Body clock constant throughout life
Young people have the same body clock length as the elderly
Scientists have calculated that the body's internal clock works on a cycle of 24 hours and 11 minutes.
Previous studies had concluded that the body, or circadian, clock worked on a 25-hour period that shortened with age.
But a study, funded by the US National Institutes for Health, has found both older and younger healthy people share the same circadian period of a little more than 24 hours. This is despite the fact that older people tend to wake up earlier.
Elderly people tend to get up earlier
Lead study author Dr Charles Czeisler, of Harvard Medical School, said: "The circadian period averages 24 hours and 11 minutes in both young and older individuals.
"However, older people tend to arise earlier in the morning, because the hours at which they can get a good night's sleep, relative to their circadian clock, are much more restricted."
The study focused on a group of 24 men and women. The 11 younger subjects had an average age of 23.7 and the 13 older subjects had an average of 67.4.
In order to measure the length of the circadian period, investigators used a number of carefully designed techniques to minimise the factors which can inadvertently reset the circadian clock.
These include activity, exposure to indoor room light, travel and illness. Genetics may also be a factor.
Dr Andrew Monjan, of the National Institute of Aging, said that older people might wake up earlier in the day than their younger counterparts because something - perhaps light exposure - is interrupting sleep and throwing off the sleep-wake cycle.
In humans the circadian pacemaker is located deep within the brain's hypothalamus, in the suprachiasmatic nucleus, where it helps the body keep time.
It controls a number of body functions and interacts with the mechanisms controlling sleep.
For most people, studies show that the pressure to sleep builds up throughout the day and peaks around 9pm-10pm.
At this time, the body's temperature starts to drop and lowers about one degree during sleep.
As it starts to rise, around 4am, the likelihood of waking increases.
In addition, the pineal gland, located deep within the brain, produces and secretes the chemical melatonin at high levels during the night.
A number of factors can affect melatonin secretion, especially many common medications and light
. For example, a lamp turned off at bedtime can affect the stimulation of the pineal gland, and sunrise triggers the chemical process that enables a person to begin waking up.