<P> According to the activation - synthesis hypothesis proposed by Robert McCarley and Allan Hobson in 1975--1977, control over REM sleep involves pathways of "REM - on" and "REM - off" neurons in the brain stem . REM - on neurons are primarily cholinergic (i.e., involve acetylcholine); REM - off neurons activate serotonin and noradrenaline, which among other functions suppress the REM - on neurons . McCarley and Hobson suggested that the REM - on neurons actually stimulate REM - off neurons, thereby serving as the mechanism for the cycling between REM and non-REM sleep . They used Lotka--Volterra equations to describe this cyclical inverse relationship . Kayuza Sakai and Michel Jouvet advanced a similar model in 1981 . Whereas acetylcholine manifests in the cortex equally during wakefulness and REM, it appears in higher concentrations in the brain stem during REM . The withdrawal of orexin and GABA may cause the absence of the other excitatory neurotransmitters; researchers in recent years increasingly include GABA regulation in their models . </P> <P> Most of the eye movements in "rapid eye movement" sleep are in fact less rapid than those normally exhibited by waking humans . They are also shorter in duration and more likely to loop back to their starting point . About seven of such loops take place over one minute of REM sleep . Whereas in slow - wave sleep the eyes can drift apart, the eyes of the paradoxical sleeper move in tandem . These eye movements follow the ponto - geniculo - occipital waves originating in the brain stem . The eye movements themselves may relate to the sense of vision experienced in the dream, but a direct relationship remains to be clearly established . Congenitally blind people, who do not typically have visual imagery in their dreams, still move their eyes in REM sleep . An alternative explanation suggests that the functional purpose of REM sleep is for procedural memory processing, and the rapid eye movement is only a side effect of the brain processing the eye - related procedural memory . </P> <P> Generally speaking, the body suspends homeostasis during paradoxical sleep . Heart rate, cardiac pressure, cardiac output, arterial pressure, and breathing rate quickly become irregular when the body moves into REM sleep . In general, respiratory reflexes such as response to hypoxia diminish . Overall, the brain exerts less control over breathing; electrical stimulation of respiration - linked brain areas does not influence the lungs, as it does during non-REM sleep and in waking . The fluctuations of heart rate and arterial pressure tend to coincide with PGO waves and rapid eye movements, twitches, or sudden changes in breathing . </P> <P> Erections of the penis (nocturnal penile tumescence or NPT) normally accompany REM sleep in rats and humans . If a male has erectile dysfunction (ED) while awake, but has NPT episodes during REM, it would suggest that the ED is from a psychological rather than a physiological cause . In females, erection of the clitoris (nocturnal clitoral tumescence or NCT) causes enlargement, with accompanying vaginal blood flow and transudation (i.e. lubrication). During a normal night of sleep the penis and clitoris may be erect for a total time of from one hour to as long as three and a half hours during REM . </P>

Paradoxical sleep refers to which stage of sleep