Delayed sleep-phase syndrome (DSPS), also known as delayed sleep-phase disorder (DSPD) or delayed sleep-phase type (DSPT), is a circadian rhythm sleep disorder, a chronic disorder of the timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily rhythms, compared to the general population and relative to societal requirements. People with DSPS generally fall asleep some hours after midnight and have difficulty waking up in the morning. Often, people with the disorder report that they cannot sleep until early morning, but fall asleep at about the ...