The First-Night Effect: Why You Sleep Worse the First Night Anywhere

You Didn't Sleep Badly Because of the Pillow

You traveled. You checked in. The bed was fine, the room was cool, you were tired. And you still slept like garbage.

Then night two, in the same room, on the same pillow, you slept like a normal human being.

That's not a coincidence and it's not the mattress. It's a real, measurable phenomenon called the first-night effect, and the explanation is wild: when you sleep somewhere unfamiliar, half your brain refuses to fully clock out.

Key facts

• The first-night effect shows up as longer time-to-sleep, less REM, more awakenings, and lower sleep efficiency on night one in any new environment (Agnew et al., 1966; Toussaint et al., 1995).
• In a 2016 Brown University study, the left hemisphere of the brain stayed shallower than the right during first-night sleep, and reacted more strongly to unexpected sounds (Tamaki et al., 2016, Current Biology).
• That asymmetry vanished by night two (Tamaki et al., 2016).
• A 2022 meta-analysis of 53 studies and 1,422 healthy people confirmed the effect across nearly the entire age spectrum (Ding et al., 2022, Sleep Medicine Reviews).
• It echoes unihemispheric slow-wave sleep in dolphins and whales, which sleep with one half of the brain at a time (Lyamin et al., 2008, Neuroscience & Biobehavioral Reviews).
• Sleep labs run an "adaptation night" specifically because researchers know night one data is junk (AAST PSG Technical Guideline, 2021).

What the First-Night Effect Actually Is

The first-night effect (FNE) is the well-documented finding that people sleep worse the first night they're recorded in an unfamiliar setting, usually a sleep lab. The pattern is consistent: you take longer to fall asleep, you spend less time in REM, your sleep is more fragmented, and your overall sleep efficiency drops.

Howard Agnew and colleagues described it in a 1966 study in Psychophysiology, back when polysomnography was new and researchers were noticing that their first-night data didn't match what they got later. Toussaint and colleagues followed up in Sleep journal in 1995, confirming the same pattern in healthy adults and showing that some sleep parameters, particularly REM, kept changing for several nights before stabilizing.

A more recent paper went further. Le Bon and colleagues (2001) reported that the effect can leak into night two, especially for REM-related metrics, which is bad news for any sleep study that only includes one adaptation night.

The bottom line: night one in a new place is not representative of how you actually sleep. Researchers have known this for 60 years.

The Brown 2016 Finding: Half Your Brain Is on Guard

Here's where it gets strange.

In 2016, Masako Tamaki, Ji Won Bang, Takeo Watanabe, and Yuka Sasaki at Brown University ran a clever set of experiments and published the results in Current Biology. The paper is called "Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans" and it changed how people think about this effect.

Using polysomnography combined with neuroimaging, they measured slow-wave activity (the deep, restorative kind) in each brain hemisphere separately. On night one, the left hemisphere had significantly less slow-wave activity than the right in the default mode network. The bigger that asymmetry was, the longer it took the person to fall asleep.

Then they played random sounds while people slept. The left hemisphere produced bigger evoked responses to unexpected, "deviant" sounds than the right. People were also more likely to wake up when the strange sound came in through the ear connected to the more-awake left hemisphere (Tamaki et al., 2016).

By night two, the asymmetry was gone. Both hemispheres slept normally.

Translation: on your first night in a hotel, one half of your brain is acting as a night watchman. It's lighter sleep on purpose, listening for threats, ready to wake you up faster than the other side. You experience that as bad sleep. Your brain experiences it as not getting murdered.

The Evolutionary Backstory

The "half-asleep" trick isn't new. Other animals have taken it much further.

Dolphins, whales, and other cetaceans practice unihemispheric slow-wave sleep (USWS), where one hemisphere sleeps deeply while the other stays awake and active, typically with the corresponding eye open. The pioneering work came from Russian neuroscientists Lev Mukhametov and his colleagues in the late 1970s (Mukhametov et al., 1977), and Oleg Lyamin's group has built on it for decades since (Lyamin et al., 2008, Neuroscience & Biobehavioral Reviews).

Bottlenose dolphins switch which hemisphere sleeps roughly every 40 minutes. They essentially never have both hemispheres in deep sleep at the same time, which makes sense because they need to surface and breathe and not get eaten. Cetaceans also show no clear REM sleep, which is its own evolutionary curiosity (Lyamin et al., 2008).

Some birds do a version of this too, and so do fur seals when they're in the water but not when they're on land (Rattenborg et al., 2016, Neuroscience & Biobehavioral Reviews).

The Tamaki team's argument is that humans, despite being terrestrial primates who sleep on stable surfaces, have kept a subtle, partial version of this trick. We don't fully sleep with one eye open. But the first time we sleep in an unfamiliar place, we sort of do. The left hemisphere takes the first shift.

How Long Does It Actually Last

The folk version of the first-night effect says "you sleep badly the first night, then you're fine." The data is more annoying than that.

A 2022 meta-analysis of 53 studies and 1,422 healthy participants by Ding and colleagues in Sleep Medicine Reviews found that the bulk of the effect resolves between night one and night two. Sleep onset latency, total sleep time, sleep efficiency, REM sleep, and wake-after-sleep-onset all improve significantly. By night three, most metrics look stable.

But REM in particular can keep shifting. Le Bon et al. (2001) argued REM adaptation can extend to night four. A 2024 paper in Scientific Reports found the first-night effect even disrupts the visual-plasticity benefits of sleep and the underlying neurochemistry, not just the architecture (Tamaki et al., 2024).

And it's not just labs. A 2024 ESRS analysis showed the effect can show up over nonconsecutive unfamiliar nights, not just back-to-back ones, meaning every new hotel resets the clock.

Night 1 vs Night 2 vs Adaptation-Night Standard

Approximate values from healthy-adult lab data, paraphrased from the Ding et al. 2022 meta-analysis and Toussaint et al. 1995:

This is exactly why the American Academy of Sleep Medicine and AAST guidelines note the FNE as a confound and labs sometimes burn a whole adaptation night before the night they actually care about.

What You Can Do About It

Most of the literature is honest about this: you can't really fight your own neurobiology. You can stack the deck.

• Bring familiar cues. Same pillow, same pajamas, same bedtime playlist. Anything that says "this is normal" to your sensory systems. The travel-sleep literature is consistent on this (Walia & Mehra, 2020, Frontiers in Psychology).
• Arrive a day early when sleep matters. If a presentation, race, or surgery depends on a rested brain, that first night in the new place is the worst-case scenario. Give yourself a night two.
• Reduce noise. The Tamaki finding suggests the watchman hemisphere is mainly listening. Earplugs or white noise reduce the deviant-sound triggers that wake you up faster.
• Don't catastrophize the bad night. Frohnhofen et al. (2019) noted people often misjudge how bad first-night sleep was. The next night usually fixes it on its own.
• If first-night sleep is consistently brutal at home too, talk to your sleep doctor. That's not the FNE. That's something else.

What This Means for Sleep Tracking Accuracy

This is the part that gets glossed over.

If a single in-lab polysomnogram is the gold standard for diagnosing how you sleep, but night one in a lab is provably not how you sleep, then a one-night lab study is structurally biased. Sleep researchers have been arguing about this for years (CHEST Physician, 2022). The push toward multi-night studies isn't just about variance, it's about getting past the FNE.

The same logic applies to home tracking. If you wear a brand-new device for the first time, sleep on a sensor pad you just installed, or change your bedroom setup the day before you start measuring, you're going to get a noisy night one. Throw it out. Don't make decisions off it.

What home-based tracking actually solves is the environment part of the first-night effect. You're already sleeping in your own bed. Your hemispheres aren't on guard. There's no lab tech taping wires to your scalp. The device just records.

That's the whole pitch for contactless sleep monitoring. Komori is being built around a 60GHz radar sensor in the bedroom designed to track movement, position, and bed exits, with all processing done locally on the device. No camera, no wearable, nothing on your skin. After your own short adaptation period to the new device existing in the room, you're getting data from your normal nights, in your normal bed, with both hemispheres asleep like they're supposed to be.

That's a different baseline than a one-night lab study. Whether one is "better" depends on what you're trying to measure, but for a long-running picture of how you actually sleep, the home recording is harder to argue with.

The Takeaway

You're not fragile. You're not bad at sleeping in new places. You're the descendant of animals that survived because some part of the brain refused to fully relax around an unfamiliar campfire, and that wiring is still in there.

If you want a representative picture of your sleep, you need more than one night, and ideally not the first night. That's true for sleep clinics. It's true for travel. And it's true for any tracker.

The good news is that once you're past it, you're past it. Your hemispheres synchronize, your REM comes back, and your sleep efficiency normalizes. Until the next hotel.

Related reading:

• What actually happens while you sleep
• Why you wake up at 3 AM (and what your body is doing)
• What jet lag does to your sleep data