Circadian Variability: The Hidden Bias in Sleep Research

January 8, 2026

Mariam Abady

Brooklyn Technical High School

9th Grade

Most people know we should sleep 8 hours a night. But what if that rule was made without considering half of the population? Studies show that women are more likely to report poor sleep in relation to men, which raises an important question. Are modern sleep standards really designed for everyone?

Sleep is controlled by the circadian rhythm, the body’s internal clock. This system is regulated in a part of the brain called the suprachiasmatic nucleus (SCN), which responds mainly to light. Light signals the brain to stay alert because it inhibits the release of melatonin, an important hormone that signals the body to prepare for sleep. Disruptions in melatonin can cause circadian rhythm misalignment, which can make sleep quality and daytime functioning suffer. Importantly, circadian rhythms don’t function the same in all bodies. Biological sex plays a role in how this system is regulated. 

Sleep differences between sexes are largely influenced by hormones, especially estrogen and progesterone. Estrogen affects sleep architecture, including deep sleep and nighttime awakenings. Because estrogen levels fluctuate across menstrual cycles, sleep quality can vary throughout the month. Progesterone has a mild sedative effect because it interacts with the GABAergic system, which helps calm brain activity. During certain phases of the menstrual cycle, these effects can lead to less stable sleep quality. 

Research also shows that women tend to have earlier circadian rhythm timing than men. Their melatonin release occurs earlier in the evening, meaning their bodies are biologically ready for sleep sooner. Even though women often sleep longer than men, they are more likely to report poor sleep and insomnia. According to the National Sleep Foundation, about 65% of women reported poor sleep quality, compared to 50% of men. This creates a confusing paradox where women seem to be sleeping “enough,” yet are still feeling unrested. 

Because women were often excluded from medical research, these differences weren’t always recognized. Scientists believed women were “too complicated,” because their complex hormonal systems introduced extraneous variables. Male bodies were treated as the default. Sleep studies, diagnostic tools, and general guidelines were developed largely based on male-centered data. This lack of inclusion meant that important biological differences in sleep were understudied or misunderstood for years. It wasn’t until 1993, when the National Institute of Health passed the Revitalization Act, that researchers were required to include women in clinical studies. 

The effects of this research gap still exist. Many sleep disorders are underdiagnosed in women because symptoms don’t always match what was originally identified for men. For example, women with sleep apnea may experience fatigue or insomnia rather than loud snoring, making it harder to recognize. Sleep quality questionnaires were also designed around male sleep patterns, meaning hormone-related sleep problems may be overlooked. As a result, many women are told their sleep problems are normal or stress-related, even when biology plays a role. 

Thankfully, things are beginning to change. The National Institute of Health now requires researchers to consider sex as a biological variable. This means studies must analyze how results differ across gender, instead of just assuming one result fits everyone. Modern sleep research is beginning to include women at all life stages and further examine how hormones affect circadian rhythms. As science gets more inclusive, sleep guidelines may become more accurate and helpful to everyone. 

Sleep science is not a one-size-fits-all. By recognizing biological differences and past research bias, scientists can create sleep recommendations that actually reflect how people sleep and not just how they were once studied.