It is generally thought that the electroencephalogram of sleep stage 2 is not uniform, depending on whether sleep stage 2 evolves toward slow-wave sleep (SWS) or toward rapid eye movement (REM) sleep. We provide here further evidence of the duality of sleep stage 2 on the basis of its autonomic and hormonal background. Fourteen healthy men (aged 21-29 years) underwent 1 experimental night. Sleep and cardiac recordings were taken from 11:00 PM to 7:00 AM. Blood was sampled continuously over 10-minute periods. Autonomic activity, as inferred from heart rate variability analysis and hormone profiles, were examined with regard to the normalized hypnograms. We found a dual activity of the autonomic nervous system during sleep stage 2, with a progressive decrease in heart rate variability sympathetic indexes during the transition toward SWS contrasting with high and rather stable levels during sleep stage 2 that evolve toward REM sleep. Also, different profiles were observed in 2 major hormone systems, the activating adrenocorticotropic system and the renin-angiotensin system. Cortisol, in its active period of circadian secretion, was stable during sleep stage 2 preceding SWS and increased significantly when sleep stage 2 preceded REM sleep. For plasma renin activity, sleep stage 2 played a transitional role, initiating increasing levels that peaked during SWS and decreasing levels that reached a nadir during REM sleep. These results indicate an autonomic and hormonal duality of sleep stage 2 that is characterized by a "quiet" period preparing SWS and an "active" period preceding REM sleep. These differences may confer a fundamental role on this sleep stage in ultradian sleep regulation.
