If you drop off very early at night, and find yourself up before the larks as early as 4am, scientists might have worked out what makes you tick.
Researchers from University of California, San Francisco studied people with ‘advanced sleep phase’ - and say that the quirk of the body clock is far more common than believed.
In fact, up to one in 300 adults may suffer from ‘advanced sleep phase’, the researchers found.
Advanced sleep phase means that the body's clock, or circadian rhythm, operates on a schedule hours earlier than most people's, with a premature release of the sleep hormone melatonin and shift in body temperature.
'While most people struggle with getting out of bed at 4 or 5 a.m., people with advanced sleep phase wake up naturally at this time, rested and ready to take on the day,' said the study's senior author, Louis Ptacek, professor of neurology at the UCSF School of Medicine.
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'These extreme early birds tend to function well in the daytime but may have trouble staying awake for social commitments in the evening.'
Additionally, 'advanced sleepers' rouse more easily than others, he said, and are satisfied with an average of an extra five-to-10 minutes of sleep on non-work days, versus the 30-to-38 minutes' more sleep of their non-advanced sleeper family members.
Ptacek and his colleagues at the University of Utah and the University of Wisconsin evaluated data from patients at a sleep disorder clinic over a nine-year period.