The 2 largest groups of patients seen are those with insomnia and excessive daytime sleepiness.

The most common causes of insomnia are related to high arousal states at night, so one is unable to fall asleep even when feeling tired at night. This can happen because of poorly managed stress, mentally stimulating activities late into the night (eg. working, arguments, exercise), stimulants such as caffeine or smoking, or disease states like depression and anxiety.

Excessive daytime sleepiness is most often due to not getting enough sleep at night, which if not voluntary, is commonly due to obstructive sleep apnoea, which causes sleep fragmentation due to recurrent awakenings and prevents the patient from entering deep sleep which is most restorative.