This is a circadian disorder that does not fit neatly into other categories such as delayed sleep disorder. People with ISWRD get the right amount of sleep, but it is not uniphasic (all in one lump at night). Instead it is spread out over three or more periods though the 24-hour day. Each period is longer than a nap, lasting one to four hours.
ISWRD is more common in older adults, although it is not a normal consequence of aging. People with traumatic brain injuries are more apt to suffer it. Degeneration of cells in the suprachiasmatic nucleus related to the development of Alzheimer’s Disease could also be related to ISWRD, but scientists just present this idea as a conjecture; they have no real proof.
This disorder is not understood, although there are clues. It appears that the homeostatic process is still working in some manner because the total amount of sleep is unchanged. The length of each sleep episode appears to depend to some extent on the circadian phase. Some scientists have put together a model you can read about here. They say their model predicts shorter sleep episodes when sleep onset happens at a low temperature and that this matches observation.
Free running disorder is different, but the two disorders may share some common cause in that the brain does not respond adequately to zeitgebers – cues from the environment. The increased prevalence of the disorder in people with dementia suggests that the disabling of neurons or neuron assemblies in the brain has some part in the overall cause.
ISWRD is worth treating only if it interferes with your life. For instance, if you have to work days and cannot afford to take naps during the day, you may wish to make sure you get all your sleep at night. The tried-and-true light exposure approach works for many. Make sure to be exposed to bright light (outdoors) during the day, especially the morning. In the evening, keep away from bright lights. This can help set your circadian rhythm.
Sedatives and insomnia medicines will put patients to sleep, but they are not good treatment choices for long-term addressing ISWRD. The person with this disorder gets enough sleep – just at the “wrong” time. Making him or her sleep more at the right time can help push the circadian cycle around the clock, but it isn’t as desirable a solution as light therapy and paying attention to good sleep hygiene.
How do you know if treatment is successful? If you return to a “normal” pattern - one long sleep period at night and no serious sleeping during the day – you are “cured”. You fit in the social norms.