EFFECT OF THE SHIFT OF THE SLEEP-WAKE CYCLE ON 3 ROBUST ENDOCRINE MARKERS OF THE CIRCADIAN CLOCK

To determine the effect of a phase shift in sleep on the circadian clo ck, thyroid-stimulating hormone (TSH), cortisol, and melatonin, three robust markers of the circadian clock, were analyzed using a 10-min bl ood sampling procedure. In an initial experiment eight subjects were s tudied during two experimental sessions: once under baseline condition s with normal nighttime sleep from 2300 to 0700 (baseline) and once af ter a night of sleep deprivation followed by daytime sleep from 0700 t o 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 base line conditions, whereas melatonin and cortisol rhythms remained unaff ected. On day 2 the amplitude of the nocturnal TSH surge was reduced i n comparison to day 1, whereas the amplitudes of melatonin and cortiso l rhythms were unchanged. There was a clear phase shift in the three e ndocrine rhythms. Triiodothyronine levels were slightly higher in the morning after the first night of sleep deprivation. These results demo nstrate that 2 consecutive days of sleep shift are sufficient to affec t the timing of the commonly accepted circadian markers, suggesting th e existence of a rapid resetting effect on the circadian clock. TSH re acts in a distinctive manner to the sleep-wake cycle manipulation by m odulating the amplitude of the nocturnal surge. This amplitude modulat ion is probably an integral part of the phase-shifting mechanisms cont rolled by the circadian clock.