The impact of melatonin, melatonin analogues, caffeine, and bright light on sleep and thermoregulatory physiology

The thermoregulatory system is tied to the regulation of sleep and wakefulness. Research protocols have examined the unmasked effect of endogenous melatonin on these systems by giving exogenous doses under controlled conditions during the daytime, when endogenous levels are low. Study findings have demonstrated that exogenous melatonin improves sleep, increases peripheral heat loss, and decreases core body temperature (CBT). These thermoregulatory adjustments mimic those that occur around habitual bedtime, when endogenous melatonin levels are high. The emergences of artificial light and stimulants i.e., caffeine have impacted the behavior and physiology that normally precede sleep. Caffeine may independently impact sleep/wakefulness, or in conjunction with the thermoregulatory system. Bright light during the biological night suppresses melatonin and changes the thermoregulatory pattern that precedes nocturnal sleep; these changes may ultimately impact the sleep/wakefulness system. To improve our understanding of physiological mechanisms promoting and disrupting sleep/wakefulness, it is important to examine the connection between melatonin and the sleep/wakefulness and thermoregulatory systems, and the impact of environmental and behavioral factors on these systems. Therefore, the aims of this dissertation were to: (1) determine the effect of a melatonin receptor analogue ramelteon, on daytime sleep and body temperature, and the relationship between the two variables; (2) determine the effect of daytime exogenous melatonin on resting energy expenditure, (REE); and (3) determine the individual and compound effects of caffeine and bright light on thermoregulatory and sleep physiology at night.;Consistent with our hypotheses, (1) ramelteon significantly improved daytime sleep, lowered CBT, and increased peripheral heat loss (2) exogenous melatonin decreased REE during the daytime, and (3) caffeine delayed the nocturnal rise in peripheral heat loss, attenuated the fall in CBT, while the combination of caffeine and bright light decreased slow wave sleep and increased sleep onset latency.;These findings suggest that melatonin may play an important role in the regulation of sleep/wakefulness as evidenced by the effect of daytime ramelteon administration on sleep and thermoregulatory physiology and the effect of daytime exogenous melatonin on REE. Finally, caffeine and bright light had a negative impact on nocturnal sleep and these effects may be mediated in part by their impact on the thermoregulatory system.