19 Thus it would appear that body temperature responses to differing light intensities can vary greatly, and with such an inconsistent response it is not easy to assign changes in body temperature as the cause for the differences in muscle endurance. The negative effect that melatonin (either endogenous or exogenous) has upon mental alertness and function might also explain the decrease in work performance. Dollins et al.20 found that melatonin (1, 20, 40, or 80 mg) significantly decreased the number GS-7340 manufacturer of correct responses in auditory vigilance, response latency in reaction time, and self-reported vigor. This

group also found that melatonin increased self-reported fatigue, confusion, and sleepiness. Using a similar testing protocol, Atkinson et al.5 found that 5 mg melatonin reduced alertness and short-term memory. In addition, eight-choice reaction time was slower at different times of day after ingesting melatonin, but exogenous melatonin did not influence

perceived exertion. On the other hand, Atkinson and associates19 found selleckchem that 2.5 mg melatonin had no effect upon mental alertness of perceived exertion before or after exercise. The above responses to melatonin would suggest that melatonin could dull the drive need to perform continual lifts, and some researchers have found confirming relationships when comparing dark and bright light exposure. For instance, French et al.14 found individuals exposed to 3000 lx had higher mental capacities than when they were exposed to 100 lx. On the other hand, Saulov and Lufi21 found no difference in perceived effort, energy,

tiredness, pleasure, and satisfaction after exercising in either normal light or dim light. Unfortunately, these researchers did not quantify the light intensity levels of their two conditions.21 Finally, Ohkuwa et al.10 measured plasma epinephrine after either 5000 lx or and 50 lx exposure for 90 min and found that epinephrine was significantly lower after bright light exposure than after dim light exposure. It is difficult to ascribe the lower work output seen in this study to a reduced neural drive when dim light shows higher epinephrine levels. A third mechanism that could relate to improved work following bright light exposure relates to changes in blood flow. Aizawa and Tokura13 found that blood flow Sitaxentan increased faster with increasing temperature during almost 11.5 h of exposure in 4000 lx vs. 100 lx. Kim and Jeong 17 also found that forearm skin blood flow tended to remain steady in 700 lx, but decreased markedly in 70 lx. Melatonin supplementation, however, shows differing responses to what would be expected from dark induced endogenous melatonin production. Atkinson et al. 19 found that during exercise, 2.5 mg melatonin magnified the increase in skin blood flow. Cook et al. 22 found that 3 mg melatonin supplementation had differing influences across various vascular beds.