In the current 24-hour society, it is important to function at optimal capacity at all times of day. In this thesis, it will be investigated if (1) light can induce daytime alertness, (2) light can affect human thermoregulation, (3) light can alter sleep, and (4) time of day has an effect on physical performance. Results indicate that light effects on human subjective alertness are relatively modest during daytime. However, if one is slightly sleep deprived, bright light exposure might induce subjective alertness. Light effects on human mental performance are relatively profound. Although bright light may not induce feelings of alertness, one will perform better. Light can affect human thermoregulation. Sleep onset is associated with a quick decrease in Core Body Temperature. If one wants to facilitate sleep onset, endogenous melatonin production and the natural drop in CBT should be facilitated. Hence, one should minimize light exposure in the evening. Oral melatonin usage can induce subjective feelings of sleepiness and decrease Core Body Temperature. Both factors may facilitate shorter sleep onset latency. Time of day affects physical performance, with better results in the early evening as compared to the morning. If one has an important sports competition and needs to perform well, try to schedule this to occur in the early evening. If this is not possible, one may choose to shift the internal body clock in such a way that the game occurs in the early evening according to internal time of the athlete.