A recognized sleep disorder which accounts for about 40% of all occurances of sleep disorders, even though some are misdiagnosed as insomnia. A person suffering from the disorder finds it difficult to match the normal sleeping habits of their society; for example, a sufferer may naturally drift towards a cycle of sleeping at 2 AM and waking at 10 AM. Often, a sufferer earns the reputation of being a "night owl" or simply "not a morning person." This last issue is a key issue, as sufferers may find it difficult to meet societal expectations (e.g. functioning at work or school).

It is theorized that DSPD is caused by a disturbance between the patient's biological clock and surrounding environmental cues. As society becomes less connected to nature, we can expect to see even more occurances of this disorder.

It may also be a natural occurance; in primates, a few in every tribe are inexplicably nocturnal, even though they are a burden on the tribe (as they are unable to hunt alone). Perhaps it is actually an evolutionary advantage; by maintaining a nocturnal population, the tribe always has a few night guards who can raise an alarm if an enemy approaches by night.