Genes that regulate our body clock also influence the restorative properties of sleep, US scientists have determined.
A study of three kinds of mice, each with a distinct genetic imprint, compared their responses to sleep deprivation.
The gene expressions were shown to affect both the need for sleep and sleep length and quality.
It is hoped that the results will develop the understanding of Seasonal Affective Disorder (SAD) and bipolar disorder, which are thought to be linked to both sleep and 24-hour rhythms.
Researcher Dr Bruce O'Hara, from the University of Kentucky, said: "We still do not know why we benefit from sleep, or why we feel tired when we are 'lacking' sleep, but it seems likely that sleep serves some basic biological function for the brain such as energy restoration for brain cells or memory consolidation.
"We have found that clock gene expression in the brain is highly correlated to the build-up of sleep debt, while previous findings have linked these genes to energy metabolism. Together, this supports the idea that one function of sleep is related to energy metabolism."
The study is available to view in the online journal BMC Neuroscience.
Please click here for advice to help you find the right type of care.