Wednesday, September 1, 1999 Published at 17:55 GMT 18:55 UK
Warm feet mean swift sleep
Temperature activates sleep hormones
It seems that granny was right - a hot water bottle or a pair of bed socks are the best way to drift quickly off to sleep.
According to Swiss researchers, you are more likely to fall asleep swiftly if your hands and feet are warmer than the temperature of the bedroom.
Herbal infusions, relaxation techniques, hot baths and sleeping pills have all been put forward as answers to that wide-awake feeling.
But it appears that nothing beats the dilation of blood vessels in the hands and feet as a sure way to induce drooping eyelids.
Sleep patterns monitored
Dr Kurt Krauchi and his team at the Sleep Laboratory at Basel monitored the body temperature and functions of a group of young, healthy men as they nodded off.
In every case, they fell asleep immediately after a shift in blood flow to hands and feet.
The study, published in the journal Nature, appears to indicate that as we approach the threshold of sleep the body's temperature regulation system redistributes heat from its core to our extremities.
The researchers say that a hot water bottle at the feet may not directly act on the central nervous system to cause sleep, but it can trigger widening of the blood vessels, which in turn switches the body's sleep mechanism on.
If the extremities are cold, inhibiting the free flow of blood, the sleep hormones fail to kick in and restless insomnia prevails.
The scientists speculate that some sleep disorders associated with old age and illness may be caused by poor circulation and an inability to widen blood vessels in the hands and feet.