Diurnal differences in L-tryptophan sleep and temperature effects in the rat

Sleep/waking and EEG power spectra were investigated for 6 h periods in rats following administration of the essential amino acid L-tryptophan (40 mg/kg), the selective serotonin uptake inhibitor zimeldine (20 mg/kg), and following a combination of L-tryptophan and zimeldine. In contrast to earlier studies, L-tryptophan decreased waking and increased total slow wave sleep when administered late in the light phase (8 1/2 h after light onset). No sleep effects were seen after early light phase injections (2 h after lights on). In agreement with earlier studies, zimeldine initially increased wakefulness, followed by an increase in slow wave sleep-2. REM sleep was abolished after zimeldine treatment. Zimeldine increased EEG delta activity and decreased EEG activity above 7 Hz. L-Tryptophan potentiated the zimeldine induced increase in waking only when given early in the light phase. In a separate experiment, body temperature was monitored after L-tryptophan injections in both early and late light phase. A thermogenic effect of L-tryptophan was seen in the early light phase, while the opposite was seen in the late light phase. The data indicate diurnal differences in sleep/waking and temperature effects of a physiological dose of L-tryptophan.