In his 1992 study "In short photoperiods, human sleep is biphasic", Thomas Wehr had seven healthy men confined to a room for fourteen hours of darkness daily for a month. At first the participants slept for about eleven hours, presumably making up for their sleep debt. After this the subjects began to sleep much as people in pre-industrial times were claimed to have done. They would sleep for about four hours, wake up for two to three hours, then go back to bed for another four hours. They also took about two hours to fall asleep.