![]() ![]() What is most important in the correlation between the duration of a REM sleep episode and the inter-REM interval is essentially the duration of the inter-REM interval itself, not the relative proportions of its NREM and/or Wake content. From this point on, “inter-REM interval” will always refer to immediately consecutive REM sleep intervals.Īfter interventions combining total sleep deprivation and REM sleep deprivation ( Ocampo-Garcés et al., 2000), and total sleep deprivation ( Franken, 2002), we have understood that although a short-term NREM sleep-dependent regulation of REM sleep cannot be ruled out (NREM Renewal theories, see Benington and Heller, 1994), REM sleep pressure accumulates mainly in a long-term process. ![]() The main correlation (between REM sleep and the consecutive inter-REM interval) was thus confirmed by the majority of reports on multiphasic sleepers, but the balance is more uneven in monophasic humans. In humans, no clear general trend was observed with either previous or consecutive episodes ( Le Bon and Linkowski, 2013), while a positive correlation was found in rodents between REM sleep duration and the spectral power of subsequent NREM episodes ( Hayashi et al., 2015). However, some studies have also found correlations between REM sleep episodes and previous NREM episodes, or a mixture of the two, in rats ( Vivaldi et al., 1994 Franken, 2002), cats ( Ursin, 1970), and humans ( Hartmann, 1966 Barbato and Wehr, 1998).Ĭomparisons between the duration of REM sleep episodes and the spectral power of NREM episodes have also produced contrasting results. These findings in rats have been confirmed in other mammals: cats ( Ursin, 1970), monkeys ( Weitzman et al., 1965 Kripke et al., 1968), mice ( Le Bon et al., 2007 Weber et al., 2018), and humans ( Barbato and Wehr, 1998). The duration of a REM sleep episode is related to the duration of the immediately consecutive inter-REM interval ( Vivaldi et al., 1994), and its NREM content ( Benington and Heller, 1994). With the exception of the Energy Allocation theory, which focuses on resource optimization ( Schmidt, 2014), all credible hypotheses on the NREM-REM cycle are currently based on a correlation that was established separately by two research teams in 1994 ( Benington and Heller, 1994 Vivaldi et al., 1994). Rationale Correlations Between REM and Immediately Consecutive Intervals vs. In a recent article, I discussed these theories and presented their respective advantages and disadvantages ( Le Bon, 2020). Rodents may exhibit 100–250 cycles per 24 h. Several theories are competing to explain this relationship within what is commonly referred to as a “NREM-REM sleep cycle,” or “ultradian sleep cycle.” Humans have between 2 and 8 such cycles during the course of a night and the most common pattern is 4–5 cycles of about 90 min (see Le Bon et al., 2019, for more on this distribution). Despite intensive research, the functional relationship between REM and NREM remains a matter of great conjecture. Since the discovery of rapid eye movement sleep (REM) by Aserinsky and Kleitman (1953), we know that sleep is composed of two distinct neurophysiological states: rapid eye movement (REM) and non-rapid eye movement (NREM). I will explain why I think this hypothesis also calls into question the notion of NREM-REM sleep cycles. After the PRRP, all three options are available again (NREM, REM, and Wake). A new “asymmetrical” hypothesis is presented here, where REM sleep episodes only determine the duration of a proportional post-REM refractory period (PRRP), during which REM sleep is forbidden and the only remaining options are isolated NREM episodes or waking. However, the high variability of inter-REM intervals, especially in polyphasic sleep, argues against a simple oscillator model. The hypothesis of short-term REM sleep homeostasis, which is currently the basis of most credible theories on sleep regulation, is built upon a positive correlation between the duration of a REM sleep episode and the duration of the interval until the next REM sleep episode (inter-REM interval): the duration of REM sleep would therefore predict the duration of this interval. ![]() Since the discovery of rapid eye movement (REM) sleep ( Aserinsky and Kleitman, 1953), sleep has been described as a succession of cycles of non-REM (NREM) and REM sleep episodes. Laboratory of Psychiatric Research (ULB 266), Department of Psychiatry, Cliniques Universitaires de Bruxelles, Université libre de Bruxelles (ULB), Bruxelles, Belgium. ![]()
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