There is no medical rule that says men must or must not wear underwear to bed. The choice comes down to two things your body is doing while you sleep: regulating temperature and managing moisture. For most men, either option is perfectly fine, but the evidence leans slightly toward loose-fitting underwear or none at all, particularly if fertility is a concern. The reasons are more interesting than you might expect, and they go well beyond simple comfort.
Why Temperature Matters More at Night Than You Think
Your body actively cools itself as part of falling asleep. In the hours before sleep onset, blood vessels in your hands and feet dilate, releasing heat from your core. This drop in core body temperature is not just a side effect of lying still; research suggests it actively triggers sleep initiation and may help you reach deeper sleep stages more quickly.1PubMed. Nighttime drop in body temperature: a physiological trigger for sleep onset? The same pattern plays out across the body’s skin surfaces: skin temperatures rise as core temperature falls, because blood is being shunted outward to dump heat.2PubMed. Distal skin vasodilation in sleep preparedness, and its impact on thermal status in preterm neonates
Anything that traps heat close to your body can interfere with this process. A snug pair of briefs adds an insulating layer right over one of the warmest regions of your body, the groin and inner thighs. Fabric type compounds the effect. A study examining sleepwear fiber types under warm conditions found that material choice influenced how quickly people fell asleep, with wool outperforming cotton and polyester for older adults in particular.3Nature and Science of Sleep. The impact of sleepwear fiber type on sleep quality under warm ambient conditions The study was looking at full sleepwear rather than underwear alone, but the principle is the same: fabrics that manage heat and moisture more efficiently lead to less sleep disruption.
This does not mean wearing underwear to bed will ruin your sleep. In a cool bedroom with breathable fabric, the effect is negligible. But if you tend to sleep hot, or your bedroom stays warm, ditching underwear removes one source of trapped heat from the equation.
The Fertility Question
This is where the evidence is strongest and where underwear choice genuinely matters for some men. Normal sperm production requires the testes to stay about two to four degrees Celsius cooler than the rest of the body.4Reproductive BioMedicine Online. Testicular heat stress and its association with male infertility That is why they sit outside the abdominal cavity in the first place, suspended in a sac with thin skin, minimal fat, and dense sweat glands, all designed to shed heat. When scrotal temperatures rise, sperm cells at various stages of development suffer. Laboratory work has confirmed that spermatogenesis performs significantly better at 35°C than at 37°C, the body’s internal temperature.5PubMed Central. Effect of Temperature on the Development of Stages of Spermatogenesis and the Functionality of Sertoli Cells In Vitro
The margin is not enormous, and that is exactly what makes it tricky. You do not need a fever to push scrotal temperatures into a range that hampers sperm production. Even your sleeping position matters: scrotal temperatures run significantly higher when you sleep on your side compared to lying on your back, because the thighs press the scrotum against the body.6PubMed. Relationship between sleeping posture and fluctuations in nocturnal scrotal temperature Tight underwear does something similar, holding the testes closer to the body and reducing the airflow that normally helps the scrotum regulate its own temperature.
A large study of men attending a fertility center found that those who primarily wore boxers had roughly 25% higher sperm concentration and 17% higher total sperm count compared to men who wore tighter styles.7PubMed Central. Type of underwear worn and markers of testicular function among men attending a fertility center Boxer-wearers also had lower levels of follicle-stimulating hormone (FSH), a hormone the brain releases to compensate when sperm production is struggling. When the researchers adjusted for FSH, the differences in sperm counts shrank, suggesting the body was already trying to correct for the heat damage in the tighter-underwear group.
A separate cross-sectional study found that men wearing loose-fit underwear both day and night had higher semen volume, though interestingly, sperm morphology (the shape of individual sperm) was actually slightly better in the tight-fit group.8Chulalongkorn Medical Journal. Association between underpants fit and semen quality: A hospital-based cross-sectional study Other parameters like motility did not show significant differences. The picture is not perfectly clean, but the overall pattern is consistent: looser is better for the quantity side of the fertility equation.
If you and your partner are trying to conceive, or if you have been told your sperm counts are borderline, sleeping in loose boxers or nothing at all is one of the easiest interventions available. It will not override a major fertility issue, but it removes one controllable source of heat stress during the eight or so hours when your testes are pressed between your body and a mattress.
What Happens Beyond Sperm
Heat stress on the testes does not only affect sperm cells. The Leydig cells, which produce testosterone, are also sensitive to elevated temperatures. Chronic heat exposure can damage these cells and reduce testosterone synthesis. Research has shown that sustained heat stress can abnormally increase aromatase activity in Leydig cells, which converts testosterone into estradiol, tipping the hormonal balance away from testosterone production.9PubMed Central. Effects of Heat Stress-Induced Sex Hormone Dysregulation on Reproduction and Growth in Male Adolescents and Beneficial Foods This is primarily a concern with more extreme or prolonged heat exposure than a pair of briefs would cause on its own, but it reinforces the general point: the testes perform better when they can stay cool.
For most men, the testosterone impact of underwear choice during sleep is probably minimal. Your body has robust compensatory mechanisms for short-term temperature fluctuations. But if you already have low testosterone or you are combining tight nightwear with other heat sources (hot baths before bed, a memory foam mattress that traps heat, heated blankets), the effects could compound in ways that are hard to measure individually but real in aggregate.
Moisture, Bacteria, and Skin Health
The groin is one of the warmest, most moisture-prone areas of your body. During eight hours of sleep, sweat and skin oils accumulate, and the dark, warm environment is exactly what fungi and bacteria prefer. This is the ecosystem that produces jock itch, folliculitis, and general irritation.
Fabric plays a decisive role here. Synthetic underwear that does not wick moisture effectively creates a sealed humid environment against the skin. Cotton breathes better but holds moisture once it gets damp, which can prolong skin contact with sweat. Research on underwear and body microclimate has confirmed that fabric composition affects local temperature and humidity across the groin and thighs, with composite-fiber blends generally managing moisture better than single-fiber fabrics.10Thermal Science. Study on the influence of underwear on local thermal and moisture comfort of human body
Going without underwear eliminates the fabric variable entirely. Air circulates freely, moisture evaporates more quickly, and the skin stays drier. If you are prone to fungal infections or chafing in the groin area, sleeping commando is one of the simplest things you can try. It is essentially the same logic behind why dermatologists sometimes recommend going braless at night or avoiding socks in bed if you are dealing with athlete’s foot: let the skin breathe when you can.
The perineal region (the area between the scrotum and the anus) is colonized by bacteria that can cause infections when conditions favor their overgrowth. Elevated moisture and pH shifts in this area have been associated with increased colonization by certain gram-negative bacteria.11PubMed Central. The effects of antiperspirant on the perineal skin flora of patients with spinal cord injury While that particular study focused on a specific patient population, the underlying biology is the same: moisture and warmth encourage bacterial growth. Reducing both by wearing less to bed, or wearing looser and more breathable fabrics, tips the balance in a healthier direction.
When Underwear at Night Makes Sense
All of the above might sound like the answer is always to skip underwear, but there are legitimate reasons some men prefer to wear it. If you sleep in shared spaces or have young children who wander into your bed, underwear provides a practical baseline of coverage. Some men find that without underwear, friction between their skin and certain sheet fabrics causes irritation, particularly on rougher cotton weaves. And if you deal with any kind of urinary leakage, a snug pair of underwear with an absorbent liner is far more practical than going without.
Men who experience nocturnal penile tumescence (the normal erections that occur during REM sleep) sometimes report that sleeping nude leads to awkward contact with sheets or uncomfortable positioning. A loose pair of boxers or boxer briefs can provide just enough containment to avoid this without adding significant heat.
Support also matters for some men. Those with a varicocele, a hernia, or post-surgical swelling may find that some degree of scrotal support at night reduces discomfort. In these cases, the mild heat trade-off is worth the comfort and stability. The key is to treat this as a case-by-case decision rather than a universal rule.
Tight Versus Loose Matters More Than On Versus Off
The research consistently points to fit as the more important variable than whether you wear underwear at all. The fertility study cited earlier compared boxer-wearers to men wearing briefs, bikinis, or jockeys, and the differences were substantial.7PubMed Central. Type of underwear worn and markers of testicular function among men attending a fertility center But within the “loose” category, there is no evidence that going fully commando offers a meaningful advantage over wearing relaxed-fit boxers. The scrotum’s built-in thermoregulation works well as long as it has room to operate. Tight underwear defeats that system; loose underwear mostly stays out of its way.
If you want the benefits of airflow without going fully nude, a pair of loose cotton or modal boxers does most of the work. The fabric should be thin and breathable, the waistband should not dig in, and there should be enough room in the pouch area that you do not feel compression. Avoid polyester or nylon blends that trap heat and moisture, especially in warm climates or during summer months.
The Partner Hygiene Angle
A question that comes up less often but is worth addressing: does a man’s underwear choice at night affect his partner’s health? The connection is indirect but real. Research has found that uropathogenic strains of E. coli, the kind that cause urinary tract infections, are significantly more likely to be shared between heterosexual sex partners than harmless strains.12Oxford Academic (American Journal of Epidemiology). Uropathogenic Escherichia coli are more likely than commensal E. coli to be shared between heterosexual sex partners The groin area is a reservoir for these bacteria, and a warm, moist environment encourages their proliferation.
This does not mean wearing briefs to bed will give your partner a UTI. But maintaining a drier, cooler groin environment overnight reduces the bacterial load on your skin, which modestly reduces the risk of transferring those organisms during intimate contact. Good hygiene practices (showering before bed, clean sheets) matter far more than underwear choice here, but it is another small factor that tilts toward breathability.
What About Boys and Teenagers
Parents sometimes wonder whether underwear choice matters for boys going through puberty. The testicular thermoregulation system develops alongside the testes themselves, and there is reason to think that the developing reproductive system may be sensitive to the same heat stressors that affect adults. Research on heat-stress-induced hormonal disruption has specifically flagged adolescent males as a group where sustained heat exposure could interfere with normal reproductive development and growth.9PubMed Central. Effects of Heat Stress-Induced Sex Hormone Dysregulation on Reproduction and Growth in Male Adolescents and Beneficial Foods
Practically speaking, this does not mean parents need to police their teenager’s sleepwear. The heat generated by a pair of underwear alone is not equivalent to the chronic heat stress studied in that research. But if a boy is already sleeping under heavy blankets in a warm room with snug-fitting underwear, the combined thermal load adds up. Encouraging loose-fitting sleepwear or making it available is reasonable without needing to turn it into a conversation about fertility.
Sleeping Position and the Variables You Cannot Control
One of the underappreciated factors in all of this is sleeping position, which you have limited control over once you are actually asleep. As noted earlier, side-sleeping pushes scrotal temperatures up compared to sleeping on your back, because the thighs compress the scrotum.6PubMed. Relationship between sleeping posture and fluctuations in nocturnal scrotal temperature Interestingly, the same study found that sudden drops in scrotal temperature during the night corresponded to changes in posture, suggesting that natural tossing and turning helps the body self-correct.
This means the testes are not stuck at one temperature all night. Your body moves, positions shift, and temperatures fluctuate. Underwear adds a constant variable on top of this dynamic process: a persistent insulating layer that is there regardless of position. Removing it does not eliminate nighttime heat fluctuations, but it gives the scrotum’s own cooling mechanisms more room to work during the positions where heat naturally spikes. For back sleepers, the difference is probably negligible. For habitual side sleepers, shedding tight underwear at night is a more meaningful change.
Fabric Choices If You Do Wear Underwear to Bed
If going commando is not for you, the fabric and fit you choose can close most of the gap. Here is what to look for:
- Cotton: Breathable and widely available, but holds moisture once damp. Fine for cool, dry bedrooms. Less ideal if you sweat heavily at night.
- Modal or bamboo rayon: Better moisture-wicking than cotton, with a softer hand feel. A solid middle ground for most sleepers.
- Merino wool blends: Surprisingly effective for temperature regulation, as the sleepwear fiber research suggests. Naturally antimicrobial. The main barrier is cost and availability in underwear cuts.
- Polyester and nylon: Trap heat and moisture against the skin. Avoid for nighttime wear unless the garment has a specific moisture-wicking treatment, and even then, cotton or modal usually outperforms in a sleep context.
Fit matters as much as fabric. A cotton brief that compresses the scrotum against the body is worse for thermoregulation than a polyester boxer that hangs loosely, even though the fabric itself is less breathable. The ideal combination is a loose cut in a moisture-managing fabric. If your current nighttime underwear leaves visible elastic marks on your skin in the morning, it is too tight for overnight wear.