This paper reviews selected biological correlates and determinants of chronotype. The diurnal types are known to differ in some parameters (mainly acrophase and nadir) of many physiological circadian rhythms such as body temperature, melatonin and cortisol secretion and cardiovascular system activity. These various characteristics of circadian rhythms reflect differences between time-of-day dependent level of arousal different for morning or evening preference. One of the manifestations of the differences in circadian changes of the level of cortical arousal typical for evening or morning preference is the interaction between chronotype and time of day of auditory and visual evoked potentials amplitude. Morning and evening types differ not only in circadian aspect of functioning but in electroencephalographic parameters of sleep. The energy of slow waveforms of sleep of evening types is higher comparing to morning types. Evening types spend less time in REM sleep also. These facts are interpreted as indicators of higher homeostatic sleep need in evening types. One of the sources of individual differences in chronotype is the level of physical maturation. Prepubertal adolescents are more morning-oriented than matured adolescents. The controversial data describing naturally occurring benzodiazepines influence on chronotype was also mentioned. The genetic basis of chronotype was discussed as the last part of the article.
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