Yes — light at night affects your sleep, and it does so even after you’ve fallen asleep. Research suggests that sleeping in a moderately lit room, rather than a dark one, is enough to raise your overnight heart rate, blunt your body’s recovery signals, and change how your body handles blood sugar the next morning. Your eyelids don’t block light the way we tend to assume, and your internal clock keeps reading the room all night long.
This is a quieter problem than screens before bed, and it gets less attention. But it’s one of the easier things to change — and it matters most for the two groups Mendtide was built for: new parents navigating night feeds by lamplight, and women in perimenopause who are already waking more than they’d like.
Does light really reach you when your eyes are closed?
Yes. Closed eyelids reduce how much light gets through, but they don’t block it. Enough passes through the thin skin and the blood vessels beneath it to reach the light-sensitive cells in your retina — including the specialized cells that report to your body clock rather than producing vision.
Those clock cells are more sensitive than most people expect. In a carefully controlled study, Zeitzer and colleagues found that roughly half of the maximum melatonin-suppressing effect of light occurred at about 100 lux — a level comparable to ordinary indoor room lighting, not a floodlight (Zeitzer et al., The Journal of Physiology, 2000). Bright sunlight is on the order of 10,000 lux or more, so 100 lux feels dim to you. Your circadian system doesn’t experience it that way.
That’s the core mismatch. Subjectively, a hallway light or a lamp left on in the corner feels like nothing. Physiologically, it’s a signal that the day isn’t over.
What does sleeping with the lights on actually do?
The most striking evidence comes from a small but tightly controlled experiment at Northwestern. Healthy adults slept one night in near-darkness (under 3 lux) and one night with moderate overhead room light (100 lux). After the lit night, participants showed higher heart rate through the night, reduced heart rate variability, and measurably worse insulin sensitivity the next morning — signs that the body’s nervous system stayed more activated rather than settling into a full recovery state (Mason et al., PNAS, 2022).
What makes this finding interesting is what it didn’t show. Total sleep time barely changed. People generally didn’t report sleeping badly. The disruption was happening underneath the experience of sleep, in exactly the signals that a wearable or a phone-based tracker picks up. If your overnight heart rate and HRV look worse than the night seemed to feel, ambient light is one of the more overlooked explanations.
Longer-term observational research points in a similar direction. In the Sister Study, a large cohort of women in the United States, sleeping with a light or television on in the room was associated with a higher likelihood of weight gain and obesity over follow-up, compared with sleeping in a dark room — an association that persisted after accounting for diet, physical activity, and sleep duration (Park et al., JAMA Internal Medicine, 2019). Observational studies can’t prove cause, and people who sleep with the TV on may differ in other ways. But it lines up with the mechanism, and it’s one of the few large datasets focused specifically on women. We look at that relationship more broadly in sleep and weight.
Work from the HEIJO-KYO cohort in Japan has found similar patterns for bedroom light exposure — associations with altered melatonin output and metabolic markers (Obayashi et al., Journal of Clinical Endocrinology & Metabolism, 2013), and, in a later longitudinal analysis, with the later development of depressive symptoms (Obayashi et al., American Journal of Epidemiology, 2018).
How is this different from blue light before bed?
They’re related but not the same problem, and it’s worth separating them.
Light before bed — the phone, the tablet, the bright bathroom — mainly delays your clock and pushes back melatonin release, making it harder to fall asleep. That’s the story we cover in blue light and screens before bed, and the practical fix is about dimming and distance in the last hour or two.
Light during sleep is a different mechanism. You’re already asleep, so it isn’t about sleep onset. It’s about whether your body is allowed to reach the quiet, low-arousal state where recovery happens. Room light during sleep appears to keep the sympathetic nervous system slightly more engaged all night — which is why the effects show up in heart rate and next-morning metabolism rather than in how long it took you to drop off.
Colour matters less here than sheer brightness. A red night light is likely gentler than a white one, but a bright red light is still bright. Darkness beats clever bulb choices.
How does this affect new parents?
If you’re doing night feeds, you already know the drill: baby stirs, you turn on a lamp or the overhead light, and for the next twenty to forty minutes you and your baby are both sitting in what your body reads as early morning.
Two things follow. First, that light exposure at 2am is landing in the window when your clock is most sensitive to being shifted — so repeated bright night feeds can nudge your rhythm later or leave it unsettled, on top of the fragmentation itself. Second, once the feed is over, getting back to sleep is measurably harder if you’ve been sitting in bright light, which compounds the sleep debt that comes with parenting.
The practical version is simple and doesn’t require heroics. Use the dimmest light that lets you safely do what you need to do — a warm, low night light rather than the overhead. Keep it out of your direct line of sight; light aimed at the ceiling or the floor reaches your eyes far less than a lamp at face height. Skip the phone screen during the feed if you can, and if you can’t, put it on its lowest brightness. And once the room is dark again, resist the urge to check the time.
None of this fixes fragmented nights. It just stops you from adding an avoidable second problem on top of an unavoidable first one.
Why does this matter more in perimenopause?
For women in their 40s and 50s, sleep is already being interrupted more often — by night sweats, by early waking, by the general fragmentation that comes with shifting hormones. Every one of those wake-ups is an opportunity to flood your eyes with light.
The pattern we hear about most often looks like this: you wake up hot at 2am, you get up, you turn the bathroom light on, you change your shirt, you check your phone to see how bad the damage is, and by the time you’re back in bed you’re fully alert. The hot flash woke you. The light is part of what kept you up. This is the same loop we describe in why you wake at 3am — a small physiological blip becomes a long awakening because of what happens next.
There’s a second reason darkness earns its keep here. Melatonin production declines with age, so the signal telling your body it’s night is quieter than it used to be. Adding light at night makes a fainter signal fainter still. That doesn’t mean supplementing is the answer — we get into that in does melatonin help sleep — but it does mean protecting the signal you have is worth more now than it was at thirty. It’s one of the more concrete adjustments available within the broader picture of perimenopause and sleep.
What actually helps?
Aim for darkness during sleep, and dimness around it. In practice:
Make the room properly dark. Blackout curtains or a well-fitted blind handle streetlights and early summer sunrises. If curtains aren’t an option — a rental, a shared room, a partner who needs a light — a sleep mask does the job. In a randomized study, wearing an eye mask overnight was associated with better next-day episodic learning and alertness compared with sleeping without one (Greco et al., Sleep, 2023).
Evict the small lights. Standby LEDs on chargers, TVs, humidifiers, and air purifiers add up in a small room. A piece of electrical tape is a legitimate sleep intervention.
Keep the clock face turned away. This helps twice: less light, and less clock-watching when you wake — which is its own trap if you’re prone to waking up too early.
Use the lowest useful light at night. For bathroom trips and feeds, a dim warm night light near floor level beats the overhead switch. You need enough to be safe; you rarely need more.
Then go the other way in the morning. The flip side of protecting darkness at night is getting real light early. Bright morning light anchors your clock and strengthens the contrast between day and night — which is why morning sunlight shows up in almost every evidence-based sleep routine, and why the two habits work better together than either does alone. Both are part of the foundation described in what sleep hygiene actually means.
If you’re someone who wakes at the slightest disturbance, darkness is doing even more work for you than it is for most people — light sleepers tend to notice the difference fastest.
The calm version
Light at night is one of the few sleep variables that’s genuinely within your control, cheap to fix, and easy to overlook. Your body reads the room even with your eyes closed, so making the room dark — and using the dimmest light you can when you have to get up — protects the recovery part of sleep, not just the falling-asleep part. If you’ve been doing everything else right and your overnight numbers still look restless, this is a reasonable thing to try next. Mendtide can help you see whether a darker room actually changes your overnight heart rate and recovery over a few weeks, rather than leaving you to guess.
You don’t have to overhaul your nights. Turn one light off, tape over one LED, and let the room be as dark as it wants to be. Your body knows what to do with darkness.