Non-24-Hour Sleep-Wake Syndrome is also known as the "hypernychthemeral syndrome." Nychthemeron is the Greek term for "a night and a day," A single 24-hour cycle of light and darkness. Therefore, "hypernychthemeral" means greater than 1 night and 1 day and refers here to the longer-than-24-hour sleep-wake cycles of patients with the non-24-hour sleep-wake syndrome.

The intrinsic circadian pacemaker inside a persons body is not trained to the 24 daily cycle. This usually consists of a chronic, steady pattern of daily delays in sleep onset and wake time in an individual. It is found that most people with this syndrome are usually blind.

These patients are as free-running, normal individuals living in a time-isolation facility with no external time cues. Their average sleep-wake cycle duration is about 25 hours, in most cases noncircadian (greater than 27 hours) sleep-wake cycles occur from time to time.

After about 1 to 2 weeks these patients get a few night of normal sleep patterns of 24 hours then back to the 25 to 27 hour days. These patients also lead normal lives, their mental and physical states are not altered at all even though their sleep patterns bounce back and forth.