How much sleep do we need?

There is increasing concern for sleeplessness-related risks in modern society. Some recent epidemiological data seem to support the view that many segments of the adult population have chronically inadequate sleep. On the other hand, some experts have claimed that our core, basic amount of sleep is around 6 h per night, and that the rest of our sleep can be easily curtailed, being unnecessary to fulfill any sleep need. However, experimental data on the effects of both acute and cumulative partial sleep deprivation (PSD) consistently point out that sleep restriction has substantial negative effects on sleepiness, motor and cognitive performance and mood, as well as on some metabolic, hormonal and immunological variables. As chronic PSD may have serious long-term adverse health effects, it should be avoided in the general population. In the short-term, the effects of sleep curtailment seem to accumulate linearly, while the effects of long-term PSD should be further investigated, as the few available studies are flawed by methodological weaknesses. On the other hand, there is evidence that extending sleep by 2-3 h beyond the norm produces only marginal benefits for an average individual. Finally, it is underlined that, as large individual differences do exist in the need for sleep, the search for the sleep need may be vain. A somnotypology, taking into account age, gender and the position in both the sleep-alert and the morningness-eveningness continuum, should help in the search for the actual individual sleep need. (C) 2001 Harcourt Publishers Ltd.