Study conducted by sleep researchers at Northwestern University has shown that when animals are partially sleep deprived over consecutive days, they no longer attempt to catch up on sleep despite an accumulating sleep deficit.

The study suggests that repeated partial sleep loss negatively affects an animal's ability to compensate for lost sleep, and that the body responds differently to chronic sleep loss than it does to acute sleep loss. Published in the Proceedings of the National Academy of Sciences, these findings shed light on a problem prevalent in industrialised nations with 24x7 societies, such as the US, where people get nearly an hour less sleep a night than they did 40 years ago.

"We now know that chronic lack of sleep has an effect on how an animal sleeps," said lead study author Fred W Turek, professor of Neurobiology and Physiology.

"The animals are getting by on less sleep but they do not try and catch up. The ability to compensate for lost sleep is itself lost, which is damaging both physically and mentally," he added.

During the study, animals were kept awake for 20 hours per day followed by four hours of sleep, over five consecutive days. Their brain wave and muscle activity patterns in order to precisely quantify sleep-wake patterns.

The animals compensated for the sleep loss by increasing the depth of sleep after the first day, which is indicative of a homeostatic response. But on the subsequent days of sleep loss, they failed to generate this compensatory response. At the end of the study, the animals were given three full days to sleep as much as they wanted. But they seldom recovered the sleep they had been deprived of.

The findings support studies that suggest that chronic partial sleep loss of even two to three hours per night could have detrimental effects on human body, leading to defects in cognitive performance, as well as cardiovascular and endocrine functions.