To determine the effect of a phase shift in sleep on the circadian clock, thyroid-stimulating hormone (TSH), cortisol, and melatonin, three robust markers of the circadian clock, were analyzed using a 10-min blood sampling procedure. In an initial experiment eight subjects were studied during two experimental sessions: once under baseline conditions with normal nighttime sleep from 2300 to 0700 (baseline) and once after a night of sleep deprivation followed by daytime sleep from 0700 to 1500 ( day 1). In a second experiment, carried out on seven subjects, the 24-h hormone profiles of the first day ( day 1) were compared with those of the second day ( day 2) of the sleep shift. During the night of sleep deprivation ( day 1) the TSH surge was higher than during baseline conditions, whereas melatonin and cortisol rhythms remained unaffected. On day 2 the amplitude of the nocturnal TSH surge was reduced in comparison to day 1, whereas the amplitudes of melatonin and cortisol rhythms were unchanged. There was a clear phase shift in the three endocrine rhythms. Triiodothyronine levels were slightly higher in the morning after the first night of sleep deprivation. These results demonstrate that 2 consecutive days of sleep shift are sufficient to affect the timing of the commonly accepted circadian markers, suggesting the existence of a rapid resetting effect on the circadian clock. TSH reacts in a distinctive manner to the sleep-wake cycle manipulation by modulating the amplitude of the nocturnal surge. This amplitude modulation is probably an integral part of the phase-shifting mechanisms controlled by the circadian clock.
Cycloheximide Inhibits Light-Induced Phase Shifting of the Circadian Clock in Neurospora
