Influence of light on exercise performance in athletes and overweight individuals

Background Athletes often need to compete at times of the day that do not meet their time of peak performance. To comply with television prime time finals often take place in the late evening, although at this time of the day there seems to be a fast decrease in performance. Bright light exposure increases alertness, reduces sleepiness and suppresses the hormone melatonin. These mechanisms induced by light exposure may prevent the time of the day related decrease in performance. In contrast to sport competitions, many exercise interventions in clinics or rehabilitation centers often take place in the morning to comply with the staffs working schedules. However, especially in the morning lower exercise intensities are chosen by most people. Light exposure has been show to increase mood and alertness, which may lead to higher training intensity and in mid- and long term to body mass reduction. Aims: The aims of this PhD project were: (1) to evaluate a possible positive dose-response relationship between bright light exposure and maximum cycling performance in athletes, (2) to investigate the effects of evening bright and blue light exposures on maximum cycling performance and (3) acoustic reaction time and handgrip strength in elite athletes and (4) to investigate the effect of morning bright light exposure on self-chosen exercise intensity and mood in overweight individuals. Methods: During this PhD project three studies were performed. In study 1 data from previous studies conducted by Prof. Schmidt-Trucksass were analyzed to evaluate a possible dose-response relationship between bright light exposure and maximum cycling performance. In the analyzed studies participants were exposed to bright light and a control light condition in a cross-over design only prior to or prior to and during a 40-minute time trial on a bicycle ergometer. The intensity of bright light and control light was identical in all studies but the studies differed in exposure durations. To compare the differences in the work performed (kJ) during the time trial within one group (i.e. one duration of light exposure) a paired t-test was used. Differences between the groups were tested with analyses of variance with the dose of light exposure (high, medium and low) and the difference in the work performed after bright and control light exposure. Based on the results of study 1 two further studies (randomized controlled trials) were planned and conducted. In study 2 (first trial; Clinicaltrials.gov ID: NCT02203539) male elite athletes performed a cardiopulmonary exercise test to assess maximum oxygen uptake (VO2max) which determines the level of fitness. One week later participants performed a reaction time task and maximum handgrip strength test before they were exposed to either bright, monochromatic blue or a control light condition in the evening for 60 minutes. The light exposure started 17 hours after each individuals’ midpoint of sleep to test all participants at the same internal time. Immediately after the light exposure participants performed the reaction time task and handgrip strength test again and then a 12-minute time trial on a bicycle ergometer. An analysis of covariance with adjustment for VO2max from the baseline test was run to compare the differences in the work performed (kJ) between the three groups. Additionally, linear regression analyses were used to estimate the effect of melanopic light exposure on melatonin suppression and end-spurt performance, which was defined as the ratio of the performance during the first and last minute of the time trial. Analyses of covariance with adjustment for the values before the light exposure were used to compare acoustic reaction time and maximum handgrip strength after the light exposure between the three groups. In study 3 (second trial; Clinicaltrials.gov ID: NCT02636335) overweight individuals performed a cardiopulmonary exercise test to assess VO2max. Two days later participants performed a 30-minute exercise session with self-chosen exercise intensity for familiarization. Three to seven days later participants were exposed to either bright or a control light condition in the morning for 30 minutes prior to and during a 30-minute exercise session with self-chosen exercise intensity on a bicycle ergometer starting at 08:00. Participants also filled out a multidimensional mood questionnaire including the three domains “good-bad”, “awake-tired”, and “calm-nervous” all of which are bipolar scales. This questionnaire was answered prior to the light exposure, after the light exposure but prior to the exercise session and after the exercise session with persisting light exposure. Analyses of covariance with adjustment for VO2max were used to compare the difference in mean power output (W) during the exercise session between the two groups. Multivariate analyses were used to test for differences in mood before the light exposure, after the light exposure and after the exercise session between the groups. Results: Publication 1: Dose-response relationship between light exposure and cycling performance in athletes [1] In athletes there was no significant difference in the work performed (kJ) during the time trial between bright light and control light in those participants that were exposed to light for only 60 minutes prior to the time trial or those participants exposed to 60 minutes prior to and during the time trial. In contrast athletes exposed to light for 120 minutes prior to and during the time trial performed significantly more work after bright light exposure. Further, there was a significant positive dose-response relationship between the duration of light exposure and the work performed over the three doses. Publication 2: Effects of bright and blue light exposure on maximum cycling performance in elite athletes [2] In elite athletes evening bright or blue light exposure for 60 minutes in duration immediately before a 12-minute time trial did not significantly increase the work performed (kJ) compared to a control condition. Athletes exposed to high doses of melanopic light showed a significantly higher performance gain during the time trial, defined as the ratio of the work performed in the first and last minute of the time trial. This was associated with a stronger decrease in melatonin. However, there were no significant changes in sleepiness, motivation or mood through the light exposure between bright or blue light compared to control light. No severe adverse events occurred in any group and minor adverse events (e.g. glare, headache) were reported as often in the bright light group as reported in the control group. Publication 3: Effects of bright and blue light exposure on simple acoustic reaction time and maximum handgrip strength in elite athletes [3] In elite athletes evening bright or blue light exposure for 60 minutes in duration immediately before a simple acoustic reaction time task and a maximum handgrip strength test did not significantly reduce reaction time (ms) or increase handgrip strength (kg) compared to a control condition. Further, the actual light intensities reaching the participants’ eyes were lower than intended according to the protocol and showed a high variation between participants. Publication 4: Effect of light exposure on self-chosen exercise intensity and mood in overweight individuals [4] In overweight individuals morning bright light exposure for 30 minutes in duration prior to and during a 30-minute exercise session did not increase self-chosen exercise intensity (mean power output in Watts) compared to a control condition. None of the three domains of the multidimensional mood questionnaire was significantly altered by light exposure. Conclusions: There is a positive dose-response relationship between the duration athletes are exposed to bright light and the maximum cycling performance in a subsequent 40-minute time trial. To increase maximum performance significantly compared to control condition a pre-exercise exposure of 120 minutes seemed to be necessary, because participants with shorter exposure durations showed no higher performance. Exposure to high doses of melanopic light in the evening improved end-spurt performance in elite athletes resulting in a potentially meaningful enhancement of performance. Although bright light did not significantly increase maximum performance further studies are recommended, because the reported difference between bright and control represents a relevant advantage in sports competitions. Acoustic reaction time and maximum handgrip strength were not improved by light exposure in elite athletes. Likewise, in overweight individuals bright light exposure in the morning did neither increase self-chosen exercise intensity in a 30-minute exercise session nor improve mood compared to exposure to control light. Athletes and overweight individuals exposed to bright light showed not more adverse events than participants in the control condition.