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Journal Title: The International journal of

neuroscience

Volume: 52 Issue: 1-2

MonthlYear: 1990Pages: 29-37

Article Author: Akerstedt T;Gillberg M

Article Title: Subjective and objective sleepiness

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¡nlel'n. J. Neuroscience, 1990, Vol. 52, pp. 29-37

Reprints available directly from the publisher

Photocopying permitted by license only

(Ç 1990 Gordon and Breach, Science Publishers, Inc.

Printed in the United Kingdom

SUBJECTIVE AND OBJECTIVE SLEEPINESS IN

THE ACTIVE INDIVIDUAL

TORBJÖRN ÂKERSTEDT

IPM & Karolinska Institute, Stockholm, Sweden

and

MA TS GILLBERG

Swedish Defence Research Establishment, Stockholm, Sweden

( Received October 16, 1989; in revised form December 11, 1989)

Eight subjects were kept awake and active overnight in a sleep lab isolated from environmental time cues.

Ambulatory EEG and EOG were continuously recorded and sleepiness ratings carried out every two hours

as was a short EEG test session with eyes open for 5 min and closed for 2 min. The EEG was subjected to

spectral analysis and the EOG was visually scored for slow rolling eye movements (SEM). Intrusions of

SEM and of alpha and theta power density during waking, open-eyed activity strongly differentiated

between high and low subjective sleepiness (the differentiation was poorer for closed eyes) and the mean

intraindividual correlations between subjective and objective sleepiness were very high. Stil, the covariation was curvilinear; physiological indices of sleepiness did not occur reliably until subjective perceptions

fell between "sleepy" and "extremely sleepy-fighting sleep"; i.e. physiological changes due to sleepiness are

not likely to occur until extreme sleepiness is encountered. The results support the notion that ambulatory

EEG/EOG changes may be used to quantify sleepiness.

Keywords: sleepiness, spectral analysis, polysomnography, EEG, EOG, ratings

Recent analyses of spectacular accidents and catastrophes suggest that sleepiness may

play an important role in such events (Mitler, et aI., 1988). Systematic investigation

of such links requires some objective measure of sleepiness. Dement & Carskadon

(1982) have operationalized the latter concept as the speed with which an individual

falls asleep, that is, proceeds from the EEG alpha activity (8-12 Hz) of relaxed

alertness to the theta activity (4-8 Hz) of the first stage of sleep. This "sleep latency

test" has become the established objective measure of sleepiness. This test, however,

requires the subject to be lying down with eyes closed in a dark room, and is obviously

ill suited for continuous monitoring of the active, open-eyed subject engaged in

normal activity. The few studies available on active subjects with open eyes clearly

indicate that alpha and theta activity of the EEG may reflect also sleepiness under

such conditions (Fruhstorfer, Langanke, Meinzer, Peter, &. Pfaff, 1977; O'Hanlon, &

Kelley, 1 977b), as well as reduced performance (Daniel, 1967; Horvath, Frantik,

Kopriva, & Meissner, 1976; O'Hanlon, & Beatty, 1977a). However, then it is the

appearance of alpha activity that signals alertness, in contrast to its disappearance, as

is the case under conditions of closed eyes (Santamaria, & Chiappa, 1987).

This work was supported by the Swedish Work Environment Fund and the Medical Research counciL.

This paper is dedicated to the memory of our friend and colleague Lars Torsvall who died on the 10th

August, 1989.

Address for correspondence: Professor Torbjörn Ákerstedt, IPM and Department for Stress Research,

Karolinska Institute, Box 60205, S-104 01 Stockholm, Sweden.

29
30 T. ÁKERSTEDT AND M. GILLBERG

METHODS

Before the te

visual analog s(

point verbally a

steps: "extreme

"sleepy-but n(

(9). The steps ii

The EEG an,

removal from (

(Princeton Insti

to i 5 seconds a

i 1/23 computer

each interval we

the conventiona

(alpha), and 13-

means across v~

The EOG wa

involving a :; 1

amplitude. The

occupied by SEJ

The main inte

with open eyes.

carried out for 1

evaluated using

was used the Wil

tests were used 1

tions students t-~

In our own work we have brought EEG/EOG methods into the field for long term

ambulatory recoiding of sleepiness in normal life situations. For this purpose we have

used small portable 4-channel Medilog tape recorders to collect data and developed

techniques for high speed spectral analysis by computer. In one study of train drivers

we found that, during the night drive, rated sleepiness increased pronouncedly, as did

alpha, theta, and delta power density as well as slow rolling eye movements (SEM)

(Torsvall, & Ákerstedt, 1987). Furthermore, the intercorrelations between these

variables were high. In particular, perceived sleepiness was closely reflected in increased alpha and theta activity, as well as in an increased number of SEMs.

However, in order to interpret EEG/EOG data from field studies in behavioraloperational terms one would need absolute criteria for critical levels of subjective and

behavioral alertness obtained under well controlled conditions. In an initial experiment (Torsvall, & Ákerstedt, 1988) we therefore describe the changes accompanying

the extreme sleepiness immediately precipitating an incident of "dozing off." This

critical point was characterized by very large increases in alpha and theta power

density as well as in slow eye movement activity. This study was strictly controlled

with respect to head (and body) position, and no subjective ratings could be obtained

since ratings would have interfered with the falling asleep process.

In the present experiment we sought to describe the EEG/EOG effects of severe

sleepiness under more natural, ad lib. conditions and to compare these to controlled

conditions. A second purpose was to study the correspondence between EEG/EOG

changes and subjective sleepiness.

Eight male subjects (age 25-47 years of age) were recruited from the institutes

involved and participated on normal paid work time (including overtime). The

experiment took place in a sleep laboratory which was isolated from all environmental

time cues (no clocks, radio, TV or natural light). The subjects arrived at the laboratory

at 2000h, were equipped with a recording device and received practice at the various

tasks. The recording was started at 2230h and continued to 1l00h the next day.

During most of this time the subjects were allowed to carry out their own preferred

activities such as reading, writing or pursuing hobbies. Lying down, sleeping, or

vigorous physical activity were not allowed. An experimenter monitored the behavior

to ensure compliance. Every two hours the subjects rated their sleepiness, carried out

a 7 min EEG/EOG test and had a small snack.

The electrophysiological recording was carried out using a portable 4-channel tape

recorder (Medilog). The EEG was obtained from a CzOz derivation and the EOG

from an oblique bipolar derivation. For the EEG/EOG test the subject was seated in

a straight-backed chair focussing on a spot at eye level at a distance of one meter.

After five minutes the subject was asked to close his eyes while remaining seated in

the same position for two additional minutes. The five minutes of open eyes was

compromise between wanting to ensure that the activation resulting from being

transferred to the test situation would have subsided and wanting to avoid exposing

the subject to a long monotonous situation. The two minutes of closed eyes was

chosen since we were interested mainly in the early effects of eye closure and since we

wanted to avoid allowing the subjects any sleep (the subjects could not be monitored

since they were recorded in an ambulatory state and were not connected to any

display equipment).

RESULTS

Variation Over 1

To describe the v,

situation from 2:

immediately befo

two measuremeni

at the 2300h tes

percentage of tim

Figure 1 show:

(Friedman L for (

correlation was (a

a regression of y

Also SEM (30, j

density from the ti

values between 05

significant variati(

For the ambula,

alpha (29, p -c .01

morning. SEM tu

presentation. No s

test conditions (v

reached r = .53 -

(t = 2.9, p ~ .05)
OBJECTIVE AND SUBJECTIVE SLEEPINESS 31

ing term

we have

:veloped

1 drivers

y, as did

s (SEM)

~n these

;d in in-

(s.

avioral-

;tive and

1 experiipanying

ff." This

.a power

)ntrolled

obtained

Before the test the subjects rated their sleepiness on two scales. One was a 100 mm

visual analog scale (VAS) from "very sleepy" to "very alert." The other one was a 9

point verbally anchored scale (Karolinska Sleepiness Scale - KSS) with the following

steps: "extremely alert" (score = 1), "alert" (3), "neither alert nor sleepy" (5),

"sleepy-but no diffculty remaining awake" (7), "Extremely sleepy-fighting sleep"

(9). The steps in between had a scale value but no verbal labeL.

The EEG and EOG were first replayed on paper and artifacts were identified for

removal from computations. The EEG was then fed through a spectrum analyzer

(Princeton Instruments) at 30 times the recording speed. The analysis epoch was set

to 15 seconds and the resulting power spectra were stored and smoothed in a PDP

11123 computer. All tapes used were calibrated using a 50 l1 V sine wave at 1.7 Hz. For

each interval we integrated the power density across each 1 Hz band as well as across

the conventional (and previously used) 0.5-3.9 Hz (delta), 4-7.9 Hz (theta), 8-12Hz

(alpha), and 13-32 Hz (beta) bands. The obtained values were then used to compute

means across various time intervals as described in the results section.

The EOG was visually analyzed for slow eye movements. These were defined as

involving a :; 1 second slow, rolling excursion of the EOG (SEM) of at least 100 l1V

amplitude. The parameter obtained was the percentage of time in each minute

occupied by SEMs.

The main interest of the present study was focussed on effects during conditions

with open eyes. For comparative purposes, however, some of the analyses were also

carried out for the 2 min test situation with eyes closed. The temporal effects were

evaluated using the Friedman "ANOV A" (Siegel, 1959). For pairwise comparisons

was used the Wilcoxon matched pairs signed ranks test (Siegel, 1959). Nonparametric

tests were used because of skewed distributions. For mean intraindividual correlations students t-test was used after z-transformation.

of severe

on trolled

~G/EOG

institutes

me). The

onmental

iboratory

ie various

next day.

preferred

:eping, or

: behavior

arried out

RESULTS

mnel tape

the EOG

; seated in

me meter.

~ seated in

eyes was

'om being

I exposing

I eyes was

d since we

monitored

:ed to any

Variation Over Time

To describe the variation over time we computed mean values for each EEGIEOG test

situation from 2300h to i 100h as well as for the last 30 minutes of ad lib. activity

immediately before each test session. The ratings were carried out in between these

two measurements. All EEG values were expressed in relation to the value obtained

at the 2300h test condition with open eyes (= i), and SEMs were expressed as

percentage of time scored.

Figure 1 shows that both types of rating scales varied significantly across time

(Friedman X2 for dr:6 = 29, p ~ .001 for both variables). Their mean intraindividual

correlation was (after z transformation) r = .93 + 0.02 (t = 40.6;p ~ .00001) with

a regression of y = .0852X + 1.2.

Also SEM (30,p ~ .00l) and alpha (25,p ~ .001) and theta (26,p ~ .001)power density from the test condition with open eyes varied significantly over time with peak

values between 0500h and 0900h (Fig. 1). Delta and beta power did not exhibit any

significant variation over time.

For the ambulatory condition there was a significant variation over time for both

alpha (29, p ~ .001) and theta (14, p ~ .05) power density with peaks during the

morning. SEM turned out to be very diffcult to score and is not included in this

presentation. No significant difference in level was found between the ambulatory and

test conditions (Wilcoxon). The mean intraindividual correlations (across time)

reached r = .53 :! 0.13 (t = 4.0, p ~ .01) for alpha power and r = .45 :! 0.15

(t = 2.9, p ~ .05) for theta power.
FIGURE 2 Mean (

and 2 minutes with c

Asterisks indicate sig

For the eyes closed condition (not in the figure) the variation over time was weakly

significant for alpha (15,p .. .05) and theta (l5,p .. .05) power density as well as for

SEM (14, p .. .05). Delta and beta power density did not vary significantly.

KSS and VAS, f(

to the verbal and

corresponds to tl

As in previous

a proportion of tl

expressed as the I

the alert conditio

followed by a strc

tion of SEM were

minutes with eyes

a strong and signi

with open eyes. 1

power the usual ei

effects were seen f

To obtain a higJ

similar to the one

the five minutes of

the open eyes coni

FIGURE 1 Mean (+ -standard error) for all variables at 2 hour intervals. In the three upper panels

filled circles represent the ambulatory conditions and open circles the test session with open eyes

In the lower panel filled circles (with broken lines) indicate results on the V AS scale and open circles results

on the KSS scale. ***p .: .001, **p .: .01, *p .: .05 for the Friedman nonparametric Anova across time.

Relations Between Subjective and Physiological Variables

To ilustrate the difference in EEGlEOG parameters between high and low sleepiness

we selected the test situations with maximum and minimum sleepiness, respectively,

and contrasted them. The scale values were 8.6(.3) vs 3.1(.6) and 89(7) vs 34(6) for the
OBJECTIVE AND SUBJECTIVE SLEEPINESS 33

10 Open eyes Closed

8

;:a. 6

ct .s 4 a.

-e

2

0

6

;:a. 4

ct

CD 2 -5 * **

0

100

Q)

'5

c:

E

Õ 50

~~

E

Q) alert (/ 0

0 2 3 4 5

Minute

FIGURE 2 Mean (+ -standard error) for EEG and EOG variables during the 5 minutes with open eyes

and 2 minutes with closed eyes for the most sleepy and most alert condition

Asterisks indicate significant deviation according to the Wilcoxon test (maximum level for 7 df p .. .0 I).

:e upper panels

Jen eyes

m circles results

)Va across time.

KSS and VAS, respectively. On the KSS scale the maximum value comes very close

to the verbal anchÖl-';"extremely sleepy, fighting sleep," whereas the minimum value

corresponds to the'rating "alert."

As in previous studies, power density for the conventional bands was expressed as

a proportion of the value with eyes open during the 2300h test situation. SEMs were

expressed as the percentage of each minute occupied by SEMs. Figure 2 shows that

the alert condition was characterized by a low alpha power density with eyes open,

followed by a strong increase upon eye closure. Theta power density and the proportion of SEM were very low throughout both the 5 minutes with eyes open and the 2

minutes with eyes closed. In contrast, the condition with maximum sleepiness showed

a strong and significant (Wilcoxon) increase in all three parameters for the 5 minutes

with open eyes. This effect disappeared somewhat during eye closure. For alpha

power the usual enhancement by eye closure actually failed to appear. No significant

effects were seen for beta or delta power density.

To obtain a higher resolution for the EEG changes we also carried out an analysis

similar to the one above but for individual 1 Hz bands. Means were computed across

the five minutes of open eyes and the two minutes of closed eyes, respectively. Under

the open eyes condition there was a significant (p .. .05) increase in power density

e was weakly

as well as for

:antly.

ow sleepiness

, respectively,

; 34(6) for the
34 T. ÁKERSTEDT AND M. GILLBERG

400 I

Closed

300 Open 'l

en

c

'"

,, 200 u

~

100

*- * '*

0

'Ii ii i.' '.1 iiiii1

8 '16 8 16 Hz

FIGURE 3 Mean (+ -standard error) deviation of the most sleepy condition from the most alert

condition for the power density spectrum in 1 Hz bands

Asterisks indicate significant difference between conditions (Wilcoxon).

for all bands from 5 to 9 Hz, the maximum increase reaching 90% for the 9 Hz band

(Figure 3). Under the closed eyes condition there was a significant increase in the 3

to 7 Hz bands and a significant decrease in the 10 and 11 Hz bands. The increase

reached a maximum of 140% at 4 Hz and the decrease a maximum of 60% atll Hz.

As a further check on the association between subjective sleepiness and the EEGl

EOG variables intraindividual correlations were computed (Table 1). All conditions

and variables showed significant mean correlations except for theta power with closed

eyes. Delta and beta power density were not entered in the computations since they

did not differ significantly between the sleepy and alert conditions. The correlations

using the V AS scale were virtually identical to those with the KSS scale.

Figure 4 shows a more detailed analysis of the relation between subjective and

physiological sleepiness. Five "bins" were constructed from the rating scales: 0-45,

46-65, 66-75, 76-85, 86-100 for the visual analog scale, corresponding to 1-4, 5-6,

7,8, and 9 for the KSS scale. For each bin the corresponding EEGlEOG data were

averaged and subjected to a Friedman nonparametric "analysis of variance" across

the five bins. All variables from the test session with open eyes and the ambulatory

condition, except theta activity during the latter, showed a significant variation across

the different levels of subjective sleepiness. However, all variables showed a significant

difference between maximum and minimum sleepiness (Wilcoxon, z ? 2.20, p ..

FIGURE 4 Mean It

pow~r with open ey'

condition, ta dito thei

Anova computed acre

.05). From the fii

or 70 is reached ,

closed eyes only S

DISCUSSION

alpha open

theta open

SEM open

alpha amb

theta amb

alpha closed

theta closed

SEM closed

.53(.15)**

.53(.11)***

.65(.04)***

.50(.04)***

.29(.09)*

.56(.20)*

.43(.19)

.57(.12)**

VAS

.53(.16)*

.52(.08)***

.61(.05)***

.49(.05)***

.53(.10)**

.51(.20)*

.43(.17)*

.46(.13)**

The EEG results (

levels of energy in

results agree with

1976; O'Hanlon (

et aI., 1988). '

In addition, We

subjective sleepine

or close to "very

appear until the sl

diffculties keepini

Thus, one should J

on the one hand ~

only varies within

At maximum slf

changes were alre~

the five minutes w~

less effcient in di

probably that the f

~his process procee

impressive since tJ

appear, or rather v

values failed to sho

eyes are more like

closed eyes.

The ambulatory

TABLE 1

Mean intraindividual correlations (standard error) between ratings and EEG/EOG variables, as well as

associated probabilities for t-test against the null hypothesis (after z-transformation).

KSS

*p .; .05, **p .; .01, ***p .; .001.
OBJECTIVE AND SUBJECTIVE SLEEPINESS 35

ost alert

--

3,0 - ...

10

00

.. 50

..............~ /0

~

2,0 -- seD

~

0. Ò ........0 *

~

,,:.~:;.:~:::::::::::::.:..::/'"

~

~

" 0

~ 1,0

40 60 80 vas

0,0

3 5 7 9 kss

Sleepiness

z band

n the 3

ncrease

: 11 Hz.

e EEGj

iditions

li closed

ice they

elations

FIGURE 4 Mean level for EEG and EOG variables at different bins of subjective sleepiness. ao = alpha

power with open eyes, to = dito theta, seo = dito sem; aa = alpha power during the ambulatory

condition, ta dito theta, Asterisks indicate level of significance according to the Friedman non parametric Anova computed across tht: 5 bins (4 df).

.05). From the figurejt is evident that no EEGjEOG changes occurred until level 7

or 70 is reached on the KSS and VAS scales, respectively. For the test session with

closed eyes only SEM showed a significant variation across bins (X2 = 13.6, P .. .01).

DISCUSSION

:Ive and

:s: 0-45,

-4, 5-6,

ita were

" across

bulatory

in across

gnificant

20, p ..

VAS

.53(.16)*

.52(.08)***

.61(.05)***

.49(.05)***

.53(.10)**

.51(.20)*

.43(.17)*

.46(. i 3)**

The EEG results clearly demonstrate that subjective sleepiness is reflected in increased

levels of energy in the alpha and theta bands under the condition of open eyes. These

results agree with earlier work (Daniel, 1967; Fruhstorfer, et aI., 1977; Horvath, et aI.,

1976; O'Hanlon, et aI., 1977a; O'Hanlon, et aI., 1977b; Torsvall, et aI., 1987; Torsvall,

et aI., 1988).

In addition, we found that the changes in the EEG did not appear until the

subjective sleepiness is considerable, i.e., not until the subject rates himself as "sleepy"

or close to "very sleepy, fighting sleep." This means that the EEG changes do not

appear until the subjective symptoms of sleepiness become manifest, for example, as

diffculties keeping ones eyes open, as an inability to focus, or as itching eyes, etc.

Thus, one should not expect to find any relation between EEG alpha or theta activity

on the one hand and subjective sleepiness or performance on the other if sleepiness

only varies within the normal daily range (from "alert" to "somewhat sleepy").

At maximum sleepiness the test session with open eyes demonstrated that the EEG

changes were already present during the first minute, although some increase across

the five minutes was seen, particularly for theta power. The eyes closed condition was

less effcient in differentiating the sleepy and alert states. One reason for this is

probably that the falling asleep process started immediately upon eye closure and that

this process proceeds with different speeds in different individuals. Still, the speed was

impressive since the normal increase in alpha activity upon eye closure failed to

appear, or rather was so rapidly replaced by theta activity that the one minute mean

values failed to show any change. The results clearly suggest that conditions with open

eyes are more likely to respond reliably to severe sleepiness than conditions with

closed eyes.

The ambulatory condition showed responses to sleepiness similar to those of the

, as well as
36 T. AKERSTEDT AND M. GILLBERG

test session. The conditions were reasonably well correlated considering the fact that

the values from the ambulatory condition were obtained after averaging across 30

minutes, thus probably involving considerable variation in sleepiness. The latter

probably also contributed to the weaker ability to differentiate between alertness and

sleepiness. This weaker ability to differentiate also raises the question of whether

averaging power values across time is the optimal approach to identifying sleepiness

in ambulatory subjects. Still, the percentage increase from baseline for both EEG

parameters (80% and 40% for alpha and theta power, respectively) were remarkably

similar to those from the study of train drivers (Torsvall, et aI., 1987). Possibly, the

values obtained could serve as rough criteria of sleepiness in field studies.

Although it is rather diffcult to determine which variable was most successful at

differentiating sleepiness from alertness it seems that SEM during the test session with

eyes open is most sensitive. The correlation with subjective sleepiness was close and

the increase during the five minutes was dramatic. Towards the end the EOG was

dominated by slow undulating patterns. Similar SEM activity was seen in our

previous laboratory study (Torsvall, et aI., 1988) and in our study of train drivers

(Torsvall, et aI., 1987).

The pattern of slow eye movements during sleepiness with "open eyes" suggest that

the session tended to become a session with alternation between open and almost

closed eyes. This, presumably, was one of the major reasons for the increase in alpha

activity. In one of our previous studies (Torsvall, & Ákerstedt, 1985) we demonstrated

that the increase in alpha power density during sleepiness corresponded to a lO-fold

increase in blink duration. Incidentially, the same study demonstrated that theta

activity was not affected by blink duration whereas delta 

activity approximately

doubled when blink duration increased from 0.2 to 2.0 seconds.

If only the test session were considered one would be tempted to draw the conclusion that SEM would be the parameter of choice to 

'identify sleepiness. SEM could

not, however, be scored reliably for the ambulatory condition since blinks and rapid

eye movement interfered. The reason that we were able to score this variable in the

study of ad lib. train drivers was probably that the drivers were seated during the

recording and were keeping their eyes focussed on the track ahead. Thus, it seems that

SEM may be diffcult to use in many ad lib. ambulatory conditions.c,

One particular problem in studies of the present type tends to be the differences

between individuals. In the present study three out of eight individuals showed

responses to sleepiness that deviated from the expected. One subject was unable to

maintain alpha activity for more than 30 to 40 seconds with closed eyes. However,

with increasing sleepiness the subject would start to oscillate between alertness and

sleepiness, leading to an oscilation in the EEG between alpha and theta activity. As

a consequence the amount of alpha activity with eyes closed would in this subject

increase with increasing sleepiness. Another subject suffered from borderline excessive

daytime sleepiness and showed much alpha activity with eyes open, even when at

maximum alertness. When maximally sleepy this subject, still with his eyes open,

shifted from alpha to theta activity and thus showed a decrease in alpha activity. A

third subject had a very large amplitude alpha activity which spread into the (formal)

theta band. When this subject closed his eyes while sleepy there was the expected

reduction of alpha activity, but also a decrease in the theta activity that had previously

been inflated by the "overflow" of alpha activity into the theta band. These observations suggest the need for individual "calibration" of sleepiness effects during the

initial parts of a study of EEG effects of sleepiness.

The need for another subdivision than the traditional into theta and alpha bands

is also supported by the analysis based on 1 Hz bands. Thus, we found that the

increase due to hi

5 to 9 Hz. During

decreased in the 1 i

might improve th

In conclusion, ,

eyes sensitively idt

ers may be used s

REFERENCES

Daniel, R. S. (1967). P

Dement, W. C., & Can

5, 56-66.

Fruhstorfer, H., Langa

indicators and ope

In R. R. Mackie (

Horvath, M., Frantik, ;

performance decre

Miller, M. M., Carskad

Ca tastrophes slee¡

O'Hanlon, J. F., & Bea

during a simulated

Mackie (Ed.), Vigi

O'Hanlon, J. F., & Kelli

drivers who perfori

Vigilance. New Yo

Santamaria, J., & Chia)

. Neurophysiology, 4,

Siegel, S. (1959). Nonpa

Torsva1\, L., & Akersted

& H. Schulz (Ed.),

Torsvall, L., & Akersted

drivers. Electroence

Torsvall, L., & Akerst;

parameters. ¡nterna
OBJECTIVE AND SUBJECTIVE SLEEPINESS 37

fact that

cross 30

ie latter

ness and

whether

leepiness

ith EEG

riarkably

;ibly, the

increase due to high sleepiness in subjects with their eyes open affected the band from

5 to 9 Hz. During the eyes closed condition power increased in the 3 to 7 Hz band and

decreased in the 10 to 11Hz band. Clearly, a more flexible approach in selecting bands

might improve the precision of EEG parameters as indicators of sleepiness.

In conclusion, we have found that the EEG and EOG during conditions of open

eyes sensitively identify states of severe sleepiness, and that at least the EEG parameters may be used successfully for this purpose in ambulatory subjects.

REFERENCES

:essful at

iion with

:lose and

~OG was

1 in our

n drivers

Daniel, R. S. (1967). Alpha and theta EEG in vigilance. Perceptual and Motor Skils, 25, 697-703.

Dement, W. c., & Carskadon, M. A. (1982). Current perspectives on daytime sleepiness: the issues. Sleep,

5, 56-66.

Fruhstorfer, H., Langanke, 1'., Meinzer, K., Peter, J. H., & Pfaff, U. (1977). Neurophysiological vigilance

indicators and operational analysis of a train vigilance monitoring device: a laboratory and field study.

In R. R. Mackie (Ed.), Vigilance. New York: Plenum Press, pp. 147-162.

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