Genetic influences on EEG sleep and the human circadian clock. A twin study

The study of neuroendocrine and sleep abnormalities in major depressive disorders has been the focus of major interest in the past few years. However, while sleep and neuroendocrine research in neuropsychiatric disorders has progressed considerably during the last few years, conceptional and methodological advances in sleep and neuroendocrine physiology are still needed for further understanding of the basic aspects of sleep and to clarify the control and significance of the temporal fluctuations of the neuroendocrine systems. In particular, identification of the genetic mechanisms governing sleep regulation are of interest. In this respect, twin studies constitute a powerful method for identifying genetic influences on human physiological variables. In a first study, we explored the sleep patterns of 26 pairs of noncohabiting normal male twins (both mono- and dizygotic). The results indicate that a significant genetic effect is found for some sleep variables. Stages 2, 4, and delta sleep as well as waking are substantially determined by genetic factors, in contrast to stage REM which seems to be mainly affected by nongenetic influences. These data thus provide consistent evidence that some aspects of human sleep are genetically determined. In a second study we analyzed the 24-hour profile of plasma cortisol in 21 pairs of male twins. The 24-hour profile of plasma cortisol is the most widely used marker of the human circadian clock: Its study offers the possibility of assessing the status of the human circadian clock and of determining whether genetic factors affect human circadian rhythmicity. In the protocol, blood was sampled every 15 min and circadian rhythmicity was characterized by measures of amplitude, phase, and overall waveshape.(ABSTRACT TRUNCATED AT 250 WORDS)