Internal Structure of Sleep Cycles in a Healthy Population

Abstract

A large body of data has been gathered on the sleep characteristics of normal subjects. The evolution of each sleep stage within each NREM/REM cycle is presented in detail, showing stage intensities minute by minute. There is a three-phase pattern in each stage intensity diagram: an initial phase of rapid change; a central phase of relative stability; and a terminating phase, again, of rapid change. The details of this pattern change progressively during the night. Throughout all cycles, there is a complementary relationship between the intensities of stage 2 sleep and the other stages that underlines the central role of stage 2 sleep in all stage transitions. Stage intensity diagrams for two groups, one group with and one group without stage 4 sleep, were compared. Subjects without stage 4 sleep tended to have a shorter duration and greater latency of stage 3 sleep. Surprisingly, cycles interrupted by abnormally long periods of continuous wake showed a negative correlation between the intensities of wake and slow wave sleep, and these interruptions did not appear to reset the cycle clock to zero. Sleep stage intensity diagrams may be useful to study the sleep patterns of populations of insomniac and depressive patients, as well as the effect of drugs on sleep.

Dynamics and Ultradian Structure of Human Sleep in Real Life

“…This means that the first deep sleep in terms of limb movement occurs mostly soon after sleep onset. This is in direct concordance with PSG-determined deep sleep (stage N3), which has its highest likelihood of occurrence after 0.2 NREM-REM cycles [32]. Sleep offset, in contrast, more likely occurs after a LIDS peak on the declining phase of the oscillation, i.e., shortly after deep sleep in terms of limb movement ( Figure 5D).…”

“…This means that movement gradually increases during sleep. Notably, the LIDS decline was rarely apparent in single sleep bouts and revealed only by averaging across bouts (a feature well known in sleep analyses [26,31,32,34]). The LIDS decline is reminiscent of the decline in the sleep EEG's slow wave power, commonly used as a proxy for sleep homeostasis [35][36][37].…”

“…However, if individual bout timelines are normalized to their LIDS period (as determined via cosine model fitting), the progressive desynchronization due to different periodicities is counteracted and the rhythmicity is maintained (Figure 1F). The distribution of the cosine-determined ultradian periods is depicted in Figure 1G and shows enrichment between 95 and 130 min (median: 110 min), durations similar to those of NREM-REM sleep cycles (e.g., [2,31,32]).…”

“…NREM episodes were delineated between the first instance of stage N2 and the first instance of stage R for each cycle, REM episodes were defined between consecutive NREM episodes. Since brief awakenings have been found not to interrupt sleep cycles [32,65], intermittent stage W or N1 within a NREM or REM episode were included in either episode unless stage W occurred in more than 20 consecutive epochs (> 10 min). In this case, the wake-interruption was marked as a WAKE episode, which either left a cycle incomplete if following a NREM episode (direct succession of NREM and REM episode was required for a complete cycle) or terminated a cycle if following a REM episode.…”

Seizure likelihood varies with day-to-day variations in sleep duration in patients with refractory focal epilepsy: A longitudinal electroencephalography investigation

“…4C). The trends were consistent across patients and the overall structure of sleep followed trends similar to those observed in healthy individuals [27]. These results thus indicate our sleep scoring methods were effective.…”