Seasonal and time-of-day variations in acute non-image forming effects of illuminance level on performance, physiology and subjective well-being

This study investigated seasonal and time-of-day dependent moderations in the strength and direction of acute diurnal non-image forming (NIF) effects of illuminance level on performance, physiology and subjective well-being. Even though there are indications for temporal variations in NIF-responsiveness to bright light, scientific insights into potential moderations by season are scarce. Moreover, it is still unknown whether effects of time-of-day influences on NIF effects are persistent throughout the year.
We employed a mixed 2 (Light: 165 vs. 1700 lx at eye level, within) x 2 (Season: autumn/winter vs. spring, between) x 2 (Time of day: morning vs. afternoon, between) mixed-model design. During each of the two 90-min experimental sessions, participants (autumn/winter: N=34; spring: N=39) completed four measurement blocks (incl. one baseline block of 120 lx at eye level) each consisting of a Psychomotor Vigilance Task (PVT) and a Backwards Digit-Span Task (BDST) including easy trials (4 – 6 digits) and difficult trials (7 – 8 digits). Heart rate (HR) and skin conductance level (SCL) were measured continuously. At the end of each lighting condition, subjective sleepiness, vitality and mood were measured.
Results revealed a clear indication for significant Light * Season interaction effects on both subjective sleepiness and vitality, which appeared only during the morning sessions. Participants felt significantly more vital and less sleepy in winter, but not in spring, after bright light exposure in the morning. In line with these subjective parameters, participants also showed significantly better PVT performance in the morning in autumn/winter but not in spring under bright light exposure. Surprisingly, for difficult working memory performance the opposite was found, namely worse performance during bright light exposure in winter but better performance when exposed to bright light in spring. Effects of bright vs. regular light exposure on physiology were quite subtle and largely nonsignificant. Season did not seem to play an important moderating role in these effects. Overall, it can be concluded that acute illuminance-induced NIF effects on subjective alertness and vitality as well as objectively measured vigilance in the morning are significantly moderated by season. Possibly, these greater illuminance-induced benefits during the morning sessions in autumn/winter compared to spring occurred due to increased responsiveness to bright light as a function of a relatively low prior light dose in autumn/winter.