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In all animals, vertebrate and invertebrate alike, one of the defining features of sleep is the "rebound", i.e., the making up for sleep debt after an acute sleep deprivation event. However, the problem of modern society is a chronic sleep loss in humans - when you loos a couple of hours of needed sleep every day.

Now, a team at Northwestern studied the effects of chronic sleep loss and, lo and behold - bad news! There is no rebound after chronic sleep deprivation.

Chronically sleep deprived? You can't make up for lost sleep:

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In the study, the researchers kept animals awake for 20 hours per day followed by a four-hour sleep opportunity, over five consecutive days. The team monitored brain wave and muscle activity patterns in order to precisely quantify sleep-wake patterns.

After the first day of sleep loss, animals compensated by increasing their intensity, or depth, of sleep, which is indicative of a homeostatic response. However, on the subsequent days of sleep loss, the animals failed to generate this compensatory response and did not sleep any more deeply or any longer than they did under non-sleep deprived conditions (baseline measurements). At the end of the study, the animals were given three full days to sleep as much as they wanted. Amazingly, they recovered virtually none of the sleep that was lost during the five-day sleep deprivation period.

The findings support what other scientists have discovered in recent experimental studies in humans. Chronic partial sleep loss of even two to three hours per night was found to have detrimental effects on the body, leading to impairments in cognitive performance, as well as cardiovascular, immune and endocrine functions. Sleep-restricted people also reported not feeling sleepy even though their performance on tasks declined.

The Northwestern team s results suggest that animals may undergo a change in their need for sleep, or in their sleep homeostat, in situations where normal sleep time is prohibited or where sleep could be detrimental for survival. An extreme but realistic example of this, says Turek, would be how animals respond to catastrophic environmental conditions, such as Hurricane Katrina. No matter how sleep deprived an animal or human may be, it would not be adaptive for the sleep homeostat to kick in and to make the animal fall sleep when it is in the midst of a flood or forest fire. Therefore, the body undergoes some change that allows it to counter its homeostatic need for sleep and to stay awake to avoid danger.

Turek and his team propose that this change in the sleep regulatory system is reflective of an allostatic response. In the short term, allostatic responses are adaptive, but when sustained on a chronic basis, such as in their study, an allostatic load will develop and lead to negative health outcomes. The allostatic load resulting from the accumulating sleep debt loops back to the sleep regulatory system itself and alters it.

Even though animals and humans may be able to adapt their sleep system to deal with repeated sleep restriction conditions, there could be negative consequences when this pattern is maintained over a long period of time, said Turek. This brings us back to the idea that repeated partial sleep restriction in humans has been linked to metabolic dysfunction and cardiovascular disease.

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