HRT changes sleep patterns to match hormonal sex

I became an early bird one day. Now my body, like clockwork, gets up before 6AM and runs out of gas by 10PM. Just a few years ago, I loved to snoozed in each morning before scrambling to get ready for my day. It just happened, and I’m not entirely sure why. Many factors likely contributed: normal aging, moving 300+ miles south (more daylight), dealing with chronic burnout, and (yes) even my HRT.

We have long known that some aspects of sleep are (on average) different between cis men and cis women. For example, cis women tend to sleep more deeply and wake up earlier than cis men. Physiologists had largely assumed these differences to be solely determined by sex chromosomes. However recent research on HRT, sleep, and trans people suggests that instead hormones (not chromosomes) are major regulators of sex differences in sleep dynamics.

A 2024 study in support of this conclusion examined chronotype, someone’s relative sleep/wake patterns with “early bird” and “night owl” as two colloquial extremes. Cis men tend to have a later chronotype (i.e. wake up later) than cis women, a difference that coincides with adolescent puberty. The researchers examined the sleep patterns of nearly 100 trans people on HRT (almost evenly split between transmascs and transfemmes), all of whom consented to participate in the study. This research was conducted by a group of Dutch scientists and published in Chronobiology International.

Participants answered sleep questionnaires and spent 7 nights wearing a portable electroencephalogram (EEG), which uses non-invasive electrodes to measure the brain’s wave-like electrical activity. From EEG data, additional measures can be extracted, and the research team was interested in how long participants slept and the halfway point of their sleep. These EEG measurements were repeated before starting HRT and after just 3 months of HRT.

The 2024 study found that chronotype predictably shifted within 3 months of HRT initiation. The midpoint of sleep (measured by MSFsc score) was 24 minutes later in transmascs and 21 minutes earlier in transfemmes, relative to the same person’s sleep before starting HRT. Notably, these changes are small (less than a half hour!), so HRT is unlikely to drive a massive recalibration of chronotype on its own. Yet, the researchers found no changes in the total length of sleep, suggesting that these shifts are truly due to alteration in chronotype rather than a secondary, general effect on sleep.

Chronotype is thought to be orchestrated by multiple brain regions like the basal ganglia, limbic system, and the hippocampus. As a whole, these brain circuits are also implicated in habit behaviors, reward processing, and emotional regulation. The somnolent influence of sex hormones is putatively mediated by these regions, raising the possibility of additional neurobiological shifts that accompany hormonal transition.

Sleep physiology can be further classified by the type of brain waves recorded by an EEG during sleep. In fact, a night of sleep can be broken down into at least four distinct stages, distinguished by the types of waves recorded by an EEG. Rapid eye movement (REM) sleep is the lightest form of sleep and is associated with dreaming. The other three phases, known as non-REM 1-3, correspond to deeper sleep and slower brain waves. The sum total of these phases is known as “sleep architecture.”

Soon after falling asleep, you slip into the deepest sleep (non-REM 3) before working your way back to REM sleep for a brief period. The cycle then repeats itself multiple times over the night. With each iteration, the time spent in deep sleep decreases and the time in REM sleep increases, before you ultimately wake up. The exact time spent in each phase of sleep varies from individual to individual, night to night, and even cycle to cycle. For example, cis men generally spend less time in deep sleep and more quickly reach REM sleep compared to cis women.

In 2023, the same Dutch research group used the same approach (7 nights of portable EEG recordings both before and 3 months after starting HRT) to analyze the sleep architecture of 73 trans people. As before, all participants consented to the study. The results were published in the journal Sleep.

This sleep architecture study found that transmascs on HRT spend less time in deep sleep and more quickly reach REM sleep after just 3 months on hormones. These changes were aligned with the sleep architecture of cis men. Testosterone, therefore, is a critical calibrator of sleep architecture by priming the brain’s electrophysiology to exhibit distinct overnight patterns.

While the 2023 study did not identify any sleep architecture changes in transfemmes, a 2011 study by German researchers documented increased non-REM sleep in transfemmes within 3 months of starting HRT (a trait associated with cis women). Given these somewhat conflicting results, more work is needed to completely parse the changes in sleep architecture among trans people.

Yet, overall these studies indicate that sex hormones (especially testosterone) regulate sleep cycles throughout adulthood. As with chronotype, sleep architecture is thought to be mediated by complex neural circuits in the brain, and sex hormones are likely acting on one or more regional nodes in this system to modulate sleep — as evidenced by the daily sleep/wake cycles of trans people on HRT.

We don’t know why we sleep, but sleep is critical for life. The brain needs sleep to function properly after a long day of being awake. During sleep, the brain consolidates memories and restores metabolic health to its ~170 billion cells. Sex hormones, not sex chromosomes, regulate multiple aspects of this vital process, namely chronotype and sleep architecture.

Sleep is not the only essential physiologic function where sex hormones drive observed differences between cis men and cis women. Heartbeats, immunity, cardiorespiratory function, and the microbiome are all governed in part by sex hormones. The indispensable roles for sex hormones in modulating vitality have only been uncovered through the physiological changes that accompany hormonal transition.

Longstanding cissexist bias in biomedical research and society falsely assumes that observed sex differences are due to actions by sex chromosomes. This is essentially what people mean when they parrot the language of “biological sex." Yet, these popular conceptions are utterly detached from reality, as evidenced by sex hormones’ primary role in driving basic physiology throughout life.

Instead, the biology of sex is much more variegated, interesting, and delicate than the simpleminded view regurgitated by anti-trans activists. Sex is a dynamic set of processes that changes with us across the span of our lives. This complex assortment of small molecule hormones, diverse cell types, and highly organized tissues are the stuff of life, and they can also be the stuff of our own choosing.

Any trans person who has ever been on HRT can regale you with stories of bodily changes that accompany hormonal transition. Our lives are evidence of sex’s messy nature. Our experiences are firsthand accounts of its tractability. Our bodies are the living proof.