Gender Issues Related to Sleep
Sleep is an active cyclic process that involves both physiological and behavioural changes. There are two major states of sleep: rapideye-movement (REM) sleep, commonly known as 'dream sleep', and non-REM (NREM) sleep. In the adult, these states cycle every 70–90 min across the night, usually beginning with NREM sleep. The state of NREM is divided into four stages: stages 1 -4, which denote the 'depth' of sleep, based upon one's ability to respond to stimulation. Stage 1 is thought to be transitional and the lightest. Stage 2 fills approximately 50% of the night and is frequently felt to be the first definite stage of sleep. The deepest stages are 3 and 4, called delta sleep, or slow-wave sleep (SWS). SWS is felt to be the most restorative stage. The depiction of these states and stages of sleep in a histogram shows the structure or architecture of one's sleep at any given time point.
Originally, scoring of sleep stages was performed visually only by comparison to standard guidelines which evaluated the EEG frequency and amplitude (Rechtschaffen and Kales, 1968) of the electroencephalogram (EEG), electro-oculogram (EOG) and electromyogram (EMG). For instance, delta sleep is represented by an EEG frequency of 0.5–4Hz and an amplitude >75μV, with steady EOG and decreased EMG, and it is further delineated into stages 3 or 4 by the percentage of a 30-s epoch which displays this frequency and amplitude. More recently, with the advent of computer scoring and spectral analysis, more definitive changes of EEG power can be documented, allowing subtle changes across and between stages of sleep to be noted.
Two main biological drives regulate the timing and amount of sleep. They are the circadian drive, or 24-h sleep/wake cycle, and the homeostatic drive, which may represent the accumulation of sleep factors or toxins produced during wake. These drives change as we age, altering the normal flow of sleep across our 24-h 'day'.
We begin to form the various stages of sleep around the fourth month of life. Sleep architecture continues to evolve throughout childhood and adolescence, and subtle differences of architecture between sexes may exist at times during this period. Sleep men reaches a fairly stable period until mid-life when gender differences begin to be more noticeable and suggest a female advantage. Sleep architecture continues to change subtly with ageing, sometimes affected by health and lifestyle.
Besides the biological changes that occur in sleep across age and gender, sleep disorders may develop, and certain of these have a higher prevalence or impact in one sex than the other. Furthermore, females' sleep is often affected by significant hormonal influences that may cause day-to-day alterations from menarche through menopause. Some women are more sensitive to these influences than others. Finally, psychosocial issues, particularly stressful lifestyle, affect one's ability to sleep well and cannot be ignored in a discussion of gender differences in sleep. Females, perhaps secondary to hormonal influences, appear to be more affected by stress than men. In addition, the psychosocial demands of mothering and caregiving frequently have a negative impact on woman's sleep.
A seminal work examining sleep in normal subjects was reported by Williams el al. (1974), who showed sleep architectural changes across ages and between genders (Table XXTV-10.1)
Table XXIV-10.1 Comparison of gender differences in sleep characteristics (Williams et al., 1974)
SWS = slow-wave sleep, TST = total sleep time, TSP = total sleep period, TIB = time in bed, REM = rapid-eye-movement
sleep, WASO = wake after sleep onset.