It is important to consider different aspects of sleep-onset, sleep-maintenance as well as daytime hypersomnolence or impaired daytime functioning.
1. Circadian Rhythm disturbances
Some people have severe sleeping problems due to shift work or interruption of the normal sleeping pattern because they have to take care of a crying baby or an old relative with dementia. Especially adolescents quite frequently do not get enough sleep if they tend to stay up until the early morning hours after a party or long computer games.
2. Psychiatric or organic causes
Insomnia is a common symptom of many psychiatric disorders like depression, manic-depressive disorders or schizophrenia. Chronic pain or heart diseases are two examples of somatic disorders with disturbances of sleep onset and maintenance.
A specialized diagnosis of sleep-related cardio-respiratory disorders with a polysomnographia in a specialized sleep center is necessary if sleep related breathing disorders, narcolepsia or other distinct forms of sleeping disorders are concerned.
3. Sleeping hygiene
There can be a lot of possible causes in the environment or your behaviour that have a negative impact on the quality of sleep.
4. Intake of sleep disturbing substances
Coffeine or nicotine can have a negative influence on your sleep. This is also true for alcohol, although many people believe alcohol would be a good "sleeping medicine". Even some medical drugs can cause insomnia.
So after all a sleep debt will be the consequence of recurrent sleept deprivation or interruption of normal sleeping paterns. If the individual does not get a sufficient amount of the restorative daily sleep that is required. There a large individual differences of the necessary amount. But something between 5 and 11 hours a night will be appropriate.