Body mass index (BMI) and obesity are some of the strongest factors affecting sleep, yet many sleep algorithms overlook them.
Elevated BMI can influence how deeply and how long you sleep, especially the all-important “restorative sleep” that helps your body fix itself overnight.
The problem is that poor sleep can create a negative cycle that makes it hard for people to improve their BMI. When you’re not getting enough deep, continuous sleep, recovery, glucose control, and hunger hormones all take a hit. You feel more tired and stressed, crave refined carbs and sugar, and over time, that makes it easier to gain weight or stay at a higher BMI.
We explore the link between BMI and sleep quality, and why it’s part of Ultrahuman’s Sleep Algorithm V2.
What is BMI?
Body Mass Index (BMI) is a system to estimate how much body fat you carry. It’s calculated by taking your weight in kilograms and dividing it by your height in meters squared. Think of it as a rough snapshot of your body’s metabolism.
While it’s not perfect and doesn’t tell the whole story, especially for those with high levels of body muscle, it’s a helpful starting point to understand your body’s relationship with weight and health.
The science behind restorative sleep and BMI
Restorative sleep is the real champion of your nightly routine. During this deep sleep phase, your body hits the reset button — healing tissues, repairing muscles, and balancing all those messy chemicals that keep you healthy and energized.
But a higher BMI can steal the spotlight from restorative sleep. When scientists looked at brain waves that occur during deep sleep, and compared them to BMIs, they found that those with higher BMIs tend to have fewer sleep waves. Since the sleep waves are linked to better memory and learning, it established a clear connection between body weight and sleep quality.
Another study found that the increase in BMI is significant across years of adult life, especially from midlife to late life (35 to 75 years). If restorative sleep improved over time, the BMI remained stable throughout the rest of an individual’s adult life.
If your BMI is elevated, chances are your body isn’t getting the full recovery it needs. And this isn’t just about feeling groggy. Over time, poor restorative sleep can throw your body’s equilibrium off, making things like weight management, heart health, and even mental wellness harder to handle.
How high BMI can affect sleep
A study showed that patients with severe obesity and obstructive sleep apnea (OSA) who underwent bariatric surgery experience important benefits. The surgery leads to significant weight loss, which in turn reduces the number of breathing interruptions during sleep (apneas), improves lung function and oxygen levels, and enhances the quality of deep and REM sleep.
In another study, researchers studied obese children and found that a body mass index (BMI) over about 29 is a strong sign that they might have severe obstructive sleep apnea (OSA). The researchers studied children’s sleep patterns using detailed monitoring and found that the higher the BMI, the more likely severe breathing problems during sleep would occur, suggesting that obese children should be carefully checked for OSA.
Together, it’s clear that restorative sleep leads to a healthier body and mind. In turn, the body and mind influence the BMI, increasing it in healthy young adults and decreasing it in elderly individuals. While related to each other, both BMI and sleep have a long-term impact on health.
Higher BMI is linked with disrupted sleep, which can exacerbate health problems and make it harder to break the cycle of poor sleep and weight gain.
How Ultrahuman’s sleep algorithm takes BMI into account
BMI – as well as age and sex – are now factors incorporated into Ultrahuman’s new Sleep Algo 2.0. It now accounts for the relationship between BMI and heart rate more precisely.
Users who improve their fitness by lowering resting HR and BMI may see noticeable sleep improvements, which can help to reinforce healthy behaviours and support weight loss goals.
Athletic or muscular individuals, whose sleep may have been under-scored previously, could see improved sleep reporting.
References
- Li W, Duan Y, Yan J, Gao H, Li X. Association between Loss of Sleep-specific Waves and Age, Sleep Efficiency, Body Mass Index, and Apnea-Hypopnea Index in Human N3 Sleep. Aging Dis. 2020 Feb 1;11(1):73-81. PMID: 32010482; PMCID: PMC6961777.
- Reither EN, Barnet JH, Palta M, Liu Y, Hagen EW, Peppard PE. Polysomnographic indicators of restorative sleep and body mass trajectories in the Wisconsin Sleep Cohort Study. Sleep. 2021 Aug 13;44(8):zsab031. PMID: 34145899; PMCID: PMC8361330.
- Qin H, Wang Y, Chen X, Steenbergen N, Penzel T, Zhang X, Li R. The efficacy of bariatric surgery on pulmonary function and sleep architecture of patients with obstructive sleep apnea and co-morbid obesity: a systematic review and meta-analysis. Surg Obes Relat Dis. 2023 Dec;19(12):1444-1457. Epub 2023 Aug 1. PMID: 37673709.
- Sukharom R, Tovichien P, Udomittipong K, Tiamduangtawan P, Chotinaiwattarakul W. Polysomnographic features of children with obesity: body mass index predict severe obstructive sleep apnea in obese children? Clin Exp Pediatr. 2025 Jan;68(1):80-90. Epub 2024 Nov 6. PMID: 39533739; PMCID: PMC11725614.
- Amiri S. Body mass index and sleep disturbances: a systematic review and meta-analysis. Postep Psychiatr Neurol. 2023 Jun;32(2):96-109. Epub 2023 Jun 28. PMID: 37497197; PMCID: PMC10367528.
- Ultrahuman Blog: Sleep Algorithm 2.0 explained: Personalized sleep scoring
