Waking hours' quality determines how quickly you doze off

The quality of wakefulness affects ease of a mammal falling asleep, a new study has suggested.

The study has helped identify two proteins that were never before linked to alertness and sleep-wake balance.

Principal author Dr. Masashi Yanagisawa, professor of molecular genetics and a Howard Hughes Medical Institute investigator at UT Southwestern said that their study supports the idea that subjective sleepiness is influenced by the quality of experiences right before bedtime.

Co-author Dr. Robert Greene, UT Southwestern professor of psychiatry and a physician at the Dallas VA Medical Center, said that the study is unique in showing that the need for sleep (called sleep homeostasis) can be separated from wakefulness both behaviourally and biochemically, meaning the two processes can now be studied individually.

The experiment featured three groups of mice with virtually identical genes. The control group slept and woke at will and followed the usual mouse pattern of sleeping during the day and being awake at night.

The two test groups were treated the same and had the same amount of sleep delay - six hours - but they were kept awake in different ways, said lead author Dr. Ayako Suzuki, a postdoctoral researcher.

The first test group's sleep was delayed by a series of cage changes. Mice are intensely curious, so each cage change was followed by an hour spent vigorously exploring the new surroundings.

Researchers kept the second group awake as gently as possible, usually by waving a hand in front of the cage or tapping it lightly whenever the mice appeared to be settling down to sleep.

Both test groups experienced the same amount of sleep deprivation, but their reactions to the different forms of alertness were striking, Dr. Yanagisawa said.

In one test, the cage-changing group took longer to fall asleep than the gentle-handling group even though an analysis of their brain waves indicated equal amounts of sleep need in both test groups.

The researchers identified two proteins that affected these responses, each linked to different aspects of sleep: phosphorylated dynamin 1 levels were linked to how long it took to fall asleep, while phosphorylated N-myc downstream regulated gene 2 protein levels tracked the amount of sleep deprivation and corresponded to the well-known brain-wave measure of sleep need, they report.

The study has been published in the Proceedings of the National Academy of Sciences.