Wakey wakey
Meanwhile, five people at the University of Colorado Boulder, in the US, were locked in a sleep laboratory for 50 days.
And as light exposure is the main way we normally control our body clock, they spent most of their incarceration in very dim light.
In a series of experiments over the month and a half, the scientists showed that an evening dose of caffeine slowed the body clock by 40 minutes.
It had roughly half the impact of three hours of bright light at bedtime.