Several twin studies examined EEG recorded during sleep. In the first study of this kind, EEG was recorded from 6 pairs of twins for 4 consecutive nights. MZ twins were highly concordant with respect to periodic changes in sleep stages, whereas DZ twins were not (Zung and Wilson, 1966). Subsequent studies reported significant genetic influences on sleep stages 2, 3, 3+4, and 4, as well as rapid eye movement (REM) density, however, no genetic influences were found for total sleep period, period of sleep, total sleep time, sleep onset latency, and REM latency (Linkowski, 1999). A more recent study also involving EEG recording over four consecutive nights suggested a very high heritability (>90%) of the spectral power of the sleep EEG in the 8 to 15.75 Hz range (De Gennaro et al., 2008). Another study investigated genetic influences on EEG composition during sleep and found significant genetic influences on some aspects of sleep architecture including the duration of sleep stage 3 and REM sleep, as well as spectral composition of non-REM sleep, particularly in delta, alpha, and theta bands (Ambrosius
