Most humans sleep from half an hour to five hours less than evolutionary forces intended them to. This means that humans have not evolved quickly enough to adapt to the pace of the present world. Because of this, most humans are living with a form of sleep debt. Sleep debt accumulates when humans do not get the amount of sleep they need for an extended period of time. This shortage of sleep builds up and breaks down the ability of humans to encounter and cope with additional restrictions on sleep that they would normally be able to handle. Specifically, the effects of less than eight hours of sleep per night accumulate as sleep debt.
A lack of sleep has various psychological and physiological consequences for humans. A sleep debt causes physical deterioration to ones health and can make one clumsy and accident-prone. It disrupts the regular function of the immune system response, hurting the body's defense strategy. It also has direct effects on thinking ability and mental efficiency. A person with a sleep debt has impaired short term memory and sometimes loses the ability to process thoughts logically. They also cannot concentrate or remember details. A lack of sleep can destroy ones psychological motivation, possibly leading to depression and other problems. It also leads to a huge drop in positive emotion and mood (Coren, 1996).
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