The acute and phase-shifting effects of artificial bright light on human physiology, performance and symptoms of jet-lag

The periodic alternation of light and darkness over the solar day has a fundamental role in synchronising human circadian rhythms. Artificial light is known to alter circadian timing depending on time of administration; early morning light can advance circadian timing while late evening light can delay circadian rhythms. Nevertheless, there is a lack of research on how manipulations of the body clock, via bright light, can enhance human performance and/or alleviate the effects of jet-lag after transmeridian travel. The doctoral programme communicated in the present thesis was designed to fill some of these gaps in knowledge. In the first experiment, the acute effects of light exposure on thermoregulation, pineal function and autonomic nervous system function (indicated by the rate pressure product) were examined. At 20:00 h, eight healthy men were exposed to a baseline period of dim light « 12 lux) for 45 min followed by 0 lux for another 60 min. Thereafter, participants were exposed to either polychromatic bright light with blue photons (2500 lux), blue photons removed (2,500 lux) or 0 lux for 30 min. Baseline data was altered for all variables in the two light conditions, suggesting an "expectancy effect" . There was evidence that light attenuated the nocturnal fall in core body temperature and rise in melatonin. These data highlight that the circadian system and other areas of the brain which are stimulated by light are still sensitive to short duration exposure and these effects are amplified by the inclusion of blue spectrum light. In a follow-up study, the effects of light exposure (blue photons included polychromatic bright light vs. no light) on subsequent early morning performance were examined under conditions of thermal stress. Participants were woken at 06:30 h. At 07:00, participants entered an environmental chamber set at 35°C and 60% relative humidity. Three 10-min bouts of exercise (55% V02max) were completed on a cycle ergometer with each bout separated with 10 min of passive recovery. Participants then completed a 10-km cycling time-trial. Core body temperature was measured every 30 s throughout the experiment using intestinal thermistors. The time of the sleep-trough in core temperature occurred approximately 1.75 h later following bright light (L) vs. no light (NL) (p = 0.07). Just prior to the time-trial, Tc was 0.27 ± OA2°C lower in L than NL (p = 0.07). The time trial was completed 1.43 ± 0.63 min quicker in L vs. NL (p = 0.001). Immediately after the time trial, intestinal temperature was 38.21 ± 0.56°C in BL compared to 38.64 ± 0.42"C in NL (p = 0.10). These data provide the first evidence that a 3D-min exposure to bright light prior to sleep can delay circadian timing to the extent that exercise performance is improved in hot conditions during the subsequent early morning. In the third study, the effects of a simulated dawn during the last 30 minutes of sleep on the subsequent dissipation of sleep inertia were examined and it was questioned whether subsequent improvements in simulated work and physical performance were mediated. Eight participants, who reported difficulty with morning waking, were administered in a random order to a control (C) and a dawn Simulation (OS) trial (starting 30 minutes prior to waking). Subjective ratings of sleep quality and alertness were obtained alongside measures of cognitive performance (addition and a reaction time tasks measured at 5, 30 and 75 minutes after waking at habitual workday times). Physical performance was also measured 35 minutes after waking using a self-paced cycling protocol. After waking in the OS condition, perceived sleep quality was 1.16 ± 0.89 (p = 0.01) points higher compared with C. Ratings of alertness were significantly higher in OS than C throughout