What Are Ultradian Rhythms and How Do They Affect Sleep? 

Most people have heard of circadian rhythms at this point: the 24 hour clock, wake up with sunlight, melatonin at night, all that. But there’s a whole other layer of biological timing that barely gets talked about, and honestly it might matter just as much for sleep quality. That’s ultradian rhythms. In this blog you will read more about ultradian rhythm, its role in the body and much more. 

What is an ultradian rhythm? 

Ultradian rhythms are biological cycles that repeat more than once within a 24 hour period. Circadian means “about a day,” Ultradian means shorter than a day, happening again and again. Heart rate variability, hormone pulses, appetite, even attention span  and a lot of these follow ultradian patterns, cycling every couple hours or sometimes every 90 minutes or so. It’s not one single rhythm either, which is part of what makes this topic messier than it first appears. They usually control the fluctuations in energy and concentration within those periods. There are several such rhythms running at once, some tied to sleep specifically and some just tied to general body regulation throughout the day.

The 90 minute thing everyone mentions but nobody explains well

If you’ve read anything about sleep cycles you’ve probably seen the number 90 minutes thrown around. That’s usually referring to the sleep-specific ultradian rhythm: the one that cycles through light sleep, deep sleep, and REM sleep repeatedly across the night. Roughly every 90 minutes, the brain moves through this sequence again. Not identical each time either. Early cycles tend to have more deep sleep, later cycles lean more REM heavy. This is the connection between sleep stages and ultradian rhythms that most articles gloss over; it’s not a flat repeating loop, it shifts in composition as the night goes on.

This is basically the role of such rhythms in sleep in a nutshell. Without this cycling structure, sleep would just be one long undifferentiated state, which actually would even work? Probably not, since different stages seem to do different restorative jobs. Deep sleep for physical repair, REM for memory and emotional processing type stuff.

How ultradian rhythms affect sleep beyond just the stage cycling?

Ultradian rhythms affect sleep in other ways like through hormone release patterns. Growth hormone, for example, pulses out in bursts tied to certain sleep stages, mostly early in the night during deep sleep. Cortisol also follows an ultradian pattern layered on top of its circadian rise and fall little pulses throughout, not a smooth curve like people imagine.

There’s also this thing called the Basic Rest-Activity Cycle (BRAC), which some researchers think continues during waking hours too like the same roughly 90 minute pattern that governs sleep stages might also govern alertness dips and rises during the day. This idea is not universally accepted, but it’s been floating around sleep science for decades and it was proposed by Kleitman. 

Natural rhythms that regulate sleep

People tend to think of sleep as governed by one clock. It’s not. There’s the circadian rhythm, the master clock in the hypothalamus responding to light and darkness. But layered on top are these natural rhythms that regulate sleep at a finer resolution; ultradian cycles doing their own thing within that bigger frame. Think of circadian as the big wave and ultradian as the ripples on top of the wave. 

This is roughly the difference between ultradian and circadian rhythms; if you want it simply: circadian is about a day long, ultradian is shorter and repeats multiple times within that day. Circadian rhythms decide when you get sleepy and when you wake up broadly speaking. Ultradian decides what happens inside the sleep period itself, structurally.

There are also infradian rhythms by the way, which go the other direction which is usually longer than a day, like menstrual cycles. Not really relevant to sleep architecture directly but worth mentioning since people confuse the terms sometimes.

How the body regulates ultradian rhythms 

How the body regulates these rhythms specifically isn’t as well mapped out as circadian regulation, which has a clear anatomical location like the suprachiasmatic nucleus (SCN), sitting in the hypothalamus, syncing to light. Ultradian regulation seems to involve interactions between different brainstem regions, particularly areas controlling REM and non-REM switching something called the reciprocal interaction model, where REM-promoting and REM-suppressing neurons essentially take turns inhibiting each other. Sounds almost too neat when written like that, and real biology is messier than any model, but that’s roughly the mechanism researchers point to.

Neurotransmitters play into it too, acetylcholine promoting REM, and other systems like norepinephrine and serotonin suppressing it, then the balance flipping back and forth. There’s probably also some involvement from the ultradian pacemaker theories tied to feeding and metabolism, but that’s a separate rabbit hole.

Connection between biological rhythms and sleep quality

The connection between biological rhythms and sleep isn’t just academic trivia, it actually shows up in how rested a person feels. If ultradian cycling gets disrupted, say from alcohol, which suppresses REM in early cycles and then rebounds it later, or from fragmented sleep due to noise or stress the structure gets thrown off even if total sleep time looks fine on paper. Someone could sleep eight hours and still feel awful because the cycling wasn’t allowed to complete properly, cycle after cycle, in the normal proportions.

This is part of why sleep trackers that just measure duration miss a lot. Duration doesn’t capture whether ultradian rhythms and healthy sleep patterns are actually lining up the way they should. Two people could sleep the same number of hours and have very different quality depending on how intact the 90 minute cycling was across the night.

Connection between biological rhythm and daytime alertness 

The connection between ultradian rhythms and daytime alertness is something people notice without necessarily naming it. That afternoon slump around 1-3 pm is partly circadian because there’s a dip in the circadian alertness curve around then. But some researchers argue there’s an ultradian component layered in as well, shorter energy and focus cycles throughout the day, maybe 90 to 120 minutes each, where attention rises and falls in waves rather than staying flat. Ever notice how focus seems to come in bursts, like you’re sharp for a stretch then suddenly foggy, then sharp again without an obvious reason? That could be these rhythms at work during waking hours, not just sleep.

Some productivity advice leans on this: work in 90 minute blocks, rest, repeat though how much of that is actual science versus repackaged Pomodoro-adjacent wellness content is genuinely unclear. Worth being skeptical of the more confident claims out there.

Final Thoughts 

Understanding ultradian rhythms doesn’t hand anyone a fix, but it reframes what “good sleep” even means. It’s not just hours logged, it’s whether the internal structure; cycle after cycle; got to play out with minimal interruption. Things like consistent bedtimes help circadian alignment, which then gives ultradian cycling a stable foundation to work from. Alcohol, frequent waking, certain medications, all mess with the cycling even when total sleep time seems okay.

There isn’t some clean five-step plan to “optimize” such rhythms, despite what some wellness sites might suggest. Mostly it comes down to protecting sleep continuity, fewer interruptions, consistent timing, less substance interference and letting the natural cycling do what it does.