A wide range of scientific studies suggest that there is a strong biological tendency for people to fall asleep in the middle of the afternoon even if they have had a full night's sleep.in a series of studies , volunteers were put in an underground room for weeks. They were completely isolated from all indicators of time .the volunteers were told to sleep whenever they wanted . Without any reference to external time measures the volunteers tended to sleep in two time periods one long session at night and another shorter one which lasted for about 2 hours in the afternoon. The naps typically happened 12 hours after the middle of the longer period of sleep