## 3 Results
_S(t)_ and _Sb(t)_: The time course of predicted highest and lowest buildups and decays of _S(t)_, respectively. The simulations suggested that, in general, the parameters of the sleep–wake regulatory processes may be practically identical in two subdivisions with RT either <7 a.m. or >=7 a.m. The only postulated differences were in sleep times on free days and in an advance in weekday waking times relative to free day waking times. The latter difference led to the difference in the timing of light exposure on weekdays and, as a consequence, to the difference in the circadian phase (0.7 h). These two corrections of preliminary simulations enabled the estimation of the differences in sleep timing caused by the phase shifts of the circadian phase in two opposite directions. One of these differences is the difference in an advance in weekday RT relative to free day RT, of either 2.0 or 3.0 h (Figure 1), and the other difference is the difference caused by the combination of this effect (i.e., the difference in the advance shift) with the effect of the difference between two chronotypes, i.e., either M-types with a smaller advance shift (2.0 h) or E-types with a larger advance shift (3.0 h) (Figure 2). The estimates of the differences caused by these bidirectional shifts enabled determination of the difference in the sleep timing of M- and E-types with the same (2.0 h) phase advance in weekday RT relative to free day RT. As shown in Table 3 (middle columns), this difference between chronotypes with a 2.0 h advance is expected to be equal to the sum of the difference caused by a larger weekday advance and the difference caused by the combination of this larger advance with the effect of chronotype on sleep timing. That is, such summation allows the subtraction of the effect of a larger advance in weekday RT in E-types for the estimation of a pure effect of the difference between M- and E-types in the internal circadian phases of their sleep-wake cycles. The summation of bidirectional differences yielded a difference of \(1.5+0.7=2.2\) h between MT with RT <7 and ET with RT \(\geq\)7 in sleep timing on free days (Table 2 and Figure 3). The sums obtained for other sleep times were the same: \(1.27+0.93=2.20\) h and \(0.71+1.49=2.20\) h for weekday and weekend bedtimes, and \(1.70+0.50=2.20\) h and \(0.81+1.39=2.20\) for weekday and weekend RTs (Table 2, upper part). These sums (Table 2, upper part) were found to be rather close to the sums obtained for the simulated lecturers' self-reports, of 2.20, 2.22, 2.16 and 2.16 h, respectively (Table 3, middle part). They were also close to the sums obtained for students' self-reports, of 1.67, 2.03, 2.04, and 2.25 h, respectively (Table 2, lower part). The same estimates could be obtained by applying another approach for the calculation of the differences in internal sleep timing of M- and E-types with the identical (2.0 h) phase advance in weekday RT relative to free day RT. These estimates (Table 3, right columns) were provided by the calculation of differences between two of four subdivisions of survey participants (i.e., in accord with two criteria, CT and weekday RT, the first division was into two CTs, M- and E-types, and the second division was into two weekday RTs, RT < 7 and RT \(\geq\) 7). The differences were calculated between M-types with early weekday RT and E-types with late weekday RT (see the illustration of simulations of their sleep times in Figure 3). The last column of Table 3 confirms the expectation of the zero differences between the estimates of sleep times based on the two methodologies for calculation of the differences in internal sleep timing of M- and E-types with the identical (2.0 h) phase advance in weekday RT relative to free day RT (left and right parts of Table 3). Figure 2: Simulations of the sleep–wake cycles of morning and evening types.