What factors determine a healthy individual's required amount of sleep? I suppose that it is well-known that different people need different amounts of sleep. I read somewhere that most people need something between 7 and 8 hours each day, though some people need only 6 hours, and (for instance) I have two friends who only need 5 hours. 

Q: Besides genetic factors, what determines a healthy individual's required amount of sleep?

For example: 


Is there a correlation between an individual's g factor and their required amount of sleep? 
Is there a correlation between an individual's weight and their required amount of sleep?


Note: I am not interested in findings related to infants or teenagers.
It is a multifactorial question dependent or correlated with a large list of variables or factors that could be grouped into:


Functions (mainly effects on neurotransmitters, also have been much studied the functions related to memory and learning).
Biological factors or bases: Indistinguishable from the previous section.
Sleep dysfunctions (terrors, somnambulism, somniloquies, nocturnal jactatio capitis, bruxism, epilepsy, mental retardation)
Disorders (initiation and maintenance disorders, apnea, myoclonus, restless legs syndrome, narcolepsy, hypersomnia, Kleine-Levin, Pickwick)
Patterns There are many sleep patterns.


It is not so easy to distinguish between health and disease since some individuals are very well adapted to patterns, dysfunctions and even some disorders.
The general intelligence factor may influence further adaptation.
The weight of the weight factor will influence to a greater extent when it is an extreme weight (mainly if there is difficulty in the breathing).

There are many manuals that address both general topics related to sleep (International Classification of Sleep Disorders, ICSD, good general books can be found in any library of psychology faculties) and very specific research (search in psycinfo).

Recommended source: Buela-Casal, G. (Comps.) (1990). Avances en la investigación del sueño y sus trastornos. Siglo Veintiuno de España Editores.