Today, average young adults report sleeping about seven to seven and one-half hours each night. Compare this to sleep patterns in 1910, before the electric lightbulb, the average person slept nine hours each night. This means that today's population sleeps one to two hours less than people did early in the century (Webb and Agnew, 1975).

Because of the advent of the lightbulb, people sleep 500 hours less each year than they used to. Unfortunately, our current "sleep diet" is significantly less than evolution intended. Most other primates (e.g., apes and monkeys) have a 24-hour sleep and activity cycle that is similar to that of humans who live in cultures where the siesta is still practiced. These animals have a long sleep at night, and a shorter sleep in the midafternoon, with a daily sleep total of about 10 hours. Humans seem to naturally need about the same amount of sleep. For instance, when the pressure of work, alarm clocks, social schedules and advanced technology is removed, people tend to sleep longer. Thus, in many less industrialized societies, the total daily sleep time is still around nine to 10 hours as it is for people when they are on unstructured holidays (Coren, 1996a).

Confirmation of these natural sleep durations comes from Palinkas, Suedfeld and Steel (1995). These researchers spent a summer above the arctic circle where there is continuous light 24 hours a day. All watches, clocks and other timekeeping devices were removed, and only the station's computers tracked the times that the team went to sleep and awakened. Individual researchers did their work, and chose when to sleep or wake according to their "body time." At the end of the experiment, they found that their overall average sleep daily time was 10.3 hours. Every member of the team showed an increase in sleep time, with the shortest logging in at 8.8 hours a day, and the longest almost 12 hours a day. This study, like many others, seems to suggest that our biological need for sleep might be closer to the 10 hours per day that is typical of monkeys and apes living in the wild, than the 7 to 7.5 hours typical of humans in today's high-tech, clock-driven lifestyle.

Psychological researchers have tended to minimize the effects of sleep insufficiency, acknowledging that society may be getting too little sleep, but treating the effects of this sleep deprivation as nothing more significant than an inconvenience which makes people feel a bit tired now and then.

This view is incorrect. Recent research suggests that each day with insufficient sleep increases our sleep debt and, when this sleep debt becomes large enough, noticeable problems appear (Coren, 1996a). These sleep-debt-related problems are most predictable at certain times of the day. This is because the efficiency of our physical and mental functions show cyclic increases and decreases in the form of circadian rhythms. While our major sleep/wakefulness rhythm has a cycle length of roughly 24 hours, there are shorter cycles as well, with the most important of these being a secondary sleep/wakefulness cycle that is around 12 hours.

Because of these cycles, the pressure to fall asleep is greatest in the morning, between 1 and 4 a.m. In addition there is a less pronounced, but still noticeable, increase in sleepiness 12 hours later, between 1 and 4 p.m. It is this afternoon low point that makes you feel sleepy after lunch, not the meal that you may have just eaten. It probably also was the original reason for the afternoon nap or siesta.

People who are operating with a sleep debt are less efficient, and this inefficiency is most noticeable when the circadian cycle is at its lowest ebb. Among the common consequences of a large sleep debt are attentional lapses, reduced short-term memory capacity, impaired judgment and the occurrence of "microsleeps."