A hormonal battle dictates when we fall asleep or wake up, according to scientists who established which part of the brain is responsible for a good night's sleep.
A joint French and Swiss research team say cells in the brain region called the ventrolateral preoptic nucleus (VLPO) holds the key to our waking and sleeping patterns.
Their research, conducted on rats, shows triangular-shaped cells in the VLPO are inhibited by hormones such as noradrenaline and serotonin when we are awake.
We then fall asleep because the VLPO cells are switched on by darkness, alcohol, warmer temperatures and other factors, they say.
In a burst of activity, the VLPO cells stop other parts of the brain releasing "wakeful" hormones.
As a result, more VLPO cells - neurons - become active, winning the war of the hormones and causing sleep.
Deepest sleep
The VLPO neurons are also highly active during the deepest sleep - the period of rapid eye movement in which the eyeballs twitch and move under the eyelids and people have vivid dreams.
The discovery, by the scientists at University Medical Centre, Geneva, the Neurobiological Laboratory for Sleeping and Wakefulness in Lyon, France, and France's National Centre for Scientific Research in Paris, was reported in the journal Nature.
Earlier research into narcolepsy - a brain disorder characterised by falling asleep anywhere, at any time of day without warning - linked it to mutations in a receptor in the brain, the hypocretin 2 receptor, involved in the transmission of nervous impulses.
Problems with sleeping - insomnia - are commonly linked to environmental and lifestyle factors, including stress, psychiatric problems, lack of exercise, and intake of caffeine and alcohol.