Circadian variation of EEG power spectra in NREM and REM sleep in humans: Dissociation from body temperature

In humans, EEG power spectra in REM and NREM sleep, as well as characterist ics of sleep spindles such as their duration, amplitude, frequency and inci dence, vary with circadian phase. Recently it has been hypothesized that ci rcadian variations in EEG spectra in humans are caused by variations in bra in or body temperature and may not represent phenomena relevant to sleep re gulatory processes. To test this directly, a further analysis of EEG power spectra - collected in a forced desynchrony protocol in which sleep episode s were scheduled to a 28-h period while the rhythms of body temperature and plasma melatonin were oscillating at their near 24-h period - was carried out. EEG power spectra were computed for NREM and REM sleep occurring between 90 -120 and 270-300 degrees of the circadian melatonin rhythm, i.e. just after the clearance of melatonin from plasma in the 'morning' and just after the 'evening' increase in melatonin secretion. Average body temperatures durin g scheduled sleep at these two circadian phases were identical (36.72 degre es C). Despite identical body temperatures, the power spectra in NREM sleep were very different at these two circadian phases. EEG activity in the low frequency spindle range was significantly and markedly enhanced after the evening increase in plasma melatonin as compared to the morning phase. For REM sleep, significant differences in power spectra during these two circad ian phases, in particular in the alpha range, were also observed. The resul ts confirm that EEG power spectra in NREM and REM sleep vary with circadian phase, suggesting that the direct contribution of temperature to the circa dian variation in EEG power spectra is absent or only minor, and are at var iance with the hypothesis that circadian variations in EEG power spectra ar e caused by variations in temperature.