Delayed sleep-phase syndrome (DSPS) is a chronic disorder of sleep timing.
People with DSPS tend to fall asleep at very late times, and also have difficulty waking up in time for school or work which begins in the morning.
Often, DSP individuals report that they cannot sleep until early morning.
Unlike insomniacs, however, they fall asleep at about the same time every night, no matter what time they go to bed.
People with DSPS have at least a normal - and often much greater than normal - ability to sleep during the morning, and sometimes in the afternoon as well.
In contrast, those with chronic insomnia do not find it much easier to sleep during the morning than at night.