As a follow-up to sex differences in migraine, Dr. Kuruvilla has edited part II. Sex Differences in Sleep, by medical writer Brooklyn A. Bradley. Please check it out. Part III, Sex Differences in Mood Disorders is coming soon!
Sex Differences in Sleep
By Brooklyn A. Bradley, Medically edited by Dr. Deena E. Kuruvilla
The second installment of the sex differences series focuses on sleep and circadian rhythm. We approximately spend a third of our day sleeping. It is a behavior characterized by decreased responsiveness to external stimuli that is related to changes in brain activity and hormones. Quality sleep is essential to maintaining the pathways in the brain that allow us to be able to function, create memories, and respond to different stimuli. The two types of sleep are known as rapid eye movement (REM) sleep and non-REM sleep. Each type can be subdivided into different stages. The stages of sleep correlate with specific brain waves and neuronal activity. Electroencephalography (EEG) can be used to measure the pattern of brain activity across awake and sleep states.
Circadian rhythms and sleep-wake homeostasis are responsible for regulating when you are awake and asleep. Circadian rhythm is also involved in body temperature, metabolism, and hormone release. These biological rhythms are the natural cycle of change in our body’s “clock.” The body’s biological clock is based on a -hour day and operates most of the circadian rhythm. Hormone release is the chemical basis of our sleep-wake cycle. Melatonin secretion begins at around 9:00 pm which makes us sleepy, and cortisol secretion begins at 6:00 am which is when we start to wake up. The sleep-wake homeostasis illustrates our individual need for sleep. The sleep drive is there to remind the body of when it needs sleep and for how long. Some factors that can impact the homeostatic sleep drive include medical conditions, medications, stress, sleep environment, and food consumption. Exposure to light is an environmental factor that is probably the most important factor, as it tells the brain whether it is day or night. It is also difficult to fall asleep in bright spaces.
Circadian system function involves the harmony of both internal and environmental factors. Daylight, meals, work schedules, friends, family, and amount of sleep are all factors in this integration. When there is a misalignment or disruption between internal and external rhythms, this can lead to circadian system dysfunction and insomnia (or other sleep disorders). A real life example of this misalignment is seen when comparing the schedules of day workers with night-shift workers. Environmental circadian disruptors include poor sleep hygiene, jet travel, shift work, sleep disorders, social jet-lag, sleep deprivation, and light pollution. Other triggers may be aging, genetic factors, neurodegenerative disease and sex. Shift work can cause circadian misalignment, which leads to disruption in the energy balances in both women and men, though through different pathways. Some of the symptoms of circadian dysfunction include fatigue, exhaustion, lethargy, decreased alertness, impaired judgment, weight changes, and much more. There has been evidence that shift workers who experience circadian misalignment are more likely to develop obesity, due to fluctuations in the hormones that control hunger signals.
A triad of sleep, weight regulation, and emotion regulation are connected to the onset of puberty. The capacity to regulate sleep fluctuates from the onset of puberty to young adulthood. These fluctuations are further categorized into positive and negative spirals. Positive spirals include healthy sleep habits, good sleep quality, positive mood, resiliency to frustration, physically active, and healthy nutritional choices. Negative spirals include staying up late worrying, irritability, low frustration tolerance, sedentary behavior, and abnormal eating patterns. Sex differences have been found not only in spirals, but also in the sleep timing of both sexes. Women tend to go to bed earlier and wake up earlier than men. Studies have found that women have an early timing of circadian rhythms as a consequence of a shorter circadian period.
Studies have shown that biological sex is a significant risk factor for insomnia. Insomnia is a sleep disorder that is characterized by difficulty falling asleep or staying asleep5. There is an association between non-healing or poor sleep quality with insomnia, and it is one of the most common sleep complaints in primary care settings. Women are more likely to report insomnia symptoms, daytime consequences of insomnia, and dissatisfaction with sleep. Women are at a 0% greater risk for insomnia throughout their lifetime compared with men. Female physiology may be the culprit behind this sex difference. Changes in ovarian steroid hormone production seen in puberty and menopause, are associated with an increased prevalence of insomnia when compared to men.
What we currently know about the mechanics of sleep is primarily sourced from studies in men. There are significant gaps in knowledge regarding the influence of sex steroids throughout the sleep cycle and basic sex differences in in sleep mechanisms. A majority of sleep studies are conducted in men or male animals. This suggests that treatment for sleep disorders is largely directed toward the male sex and male physiology. Current treatments may not target specific mechanisms in women. It is vital to compare and contrast both sleep, circadian rhythms, and incidence of sleep disorders in both the men and women. Once we are able to comprehend the biological and environmental mechanisms that influence sleep in women, we can recommend treatments that specifically benefit both sexes.
References:
- Brain Basics: Understanding Sleep | National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep. Accessed 5 Dec 2021
- Mong JA, Cusmano DM (2016) Sex differences in sleep: impact of biological sex and sex steroids. Philos Trans R Soc Lond B Biol Sci 371:20150110. https://doi.org/10.1098/rstb.2015.0110
- Circadian Rhythms. https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx. Accessed 5 Dec 2021
- Qian J, Morris CJ, Caputo R, et al (2019) Sex differences in the circadian misalignment effects on energy regulation. PNAS 116:23806–23812. https://doi.org/10.1073/pnas.1914003116
- Insomnia – Symptoms and causes – Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/insomnia/symptoms-causes/syc-20355167#:~:text=Insomnia%20is%20a%20common%20sleep,tired%20when%20you%20wake%20up. Accessed 5 Dec 2021