A new room gives the sleeping brain work it does not have at home. The mattress feels different, the air conditioner clicks on an unfamiliar schedule, and footsteps in the corridor have not yet been classified as harmless. Even when the bed is comfortable, the first night can contain more wakefulness and less settled sleep.
Sleep researchers call this the first-night effect. It is real enough to influence how studies are designed, but its cause is less tidy than the appealing idea that half the brain simply stands guard.
The first night can distort a sleep measurement
In a classic 1966 laboratory study, researchers recorded 43 people over four consecutive nights. The first night showed more wakefulness, less REM sleep, and delays in reaching some stages. Much of the disruption faded by the second night.
That creates a measurement problem. If a participant sleeps in a strange room while electrodes record every movement, is the study observing normal sleep or adaptation to the laboratory? Some protocols use an adaptation night so later recordings are less dominated by novelty.
The same logic applies outside a lab, although hotels bring extra variables. Noise, light, temperature, travel stress, jet lag, alcohol, schedule changes, and an unfamiliar pillow can all disturb sleep. “New place” is not one isolated biological switch.
Sleep is not equally deep everywhere
Sleep creates a trade-off. An animal needs restoration but becomes less available to detect danger. Some birds and marine mammals manage that trade-off with unihemispheric slow-wave sleep, allowing one hemisphere to show stronger sleep while the other remains more alert.
Some birds and marine mammals use unihemispheric slow-wave sleep; the human first-night finding was much narrower.
Humans do not normally sleep like dolphins. Yet human sleep is not one master switch. Slow-wave activity can vary across cortical regions depending on prior use, sleep pressure, development, and local conditions. That makes partial, temporary vigilance biologically plausible without implying that an entire hemisphere remains awake.
The broader article on why the human brain is never really finished describes a similar theme: brain systems change on different schedules rather than moving through one universal state together.
What the 2016 night-watch study found
In 2016, researchers studied 35 young, healthy adults across several small experiments. Participants spent two sessions in a sleep laboratory about a week apart. The team combined polysomnography with structural imaging and magnetoencephalography, which records magnetic fields associated with brain activity.
During slow-wave sleep on the first laboratory night, part of the left hemisphere’s default-mode network showed lower slow-wave activity than the corresponding right-side region. The researchers interpreted that difference as shallower sleep. Greater asymmetry was also associated with taking longer to fall asleep. The pattern was not apparent during the second session.
The 2016 study reported lower slow-wave activity in part of the left hemisphere during the first laboratory night.
The team then played sequences of tones. Most were predictable; an occasional tone differed. Sounds presented to one ear can bias processing toward the opposite hemisphere, although human auditory pathways overlap. On the first night, unusual tones directed more strongly toward the less deeply sleeping left-side network produced larger responses and were more likely to trigger arousal.
The experiment compared responses to predictable tones with occasional unusual sounds that could trigger stronger arousal.
Together, the results supported a limited night-watch mechanism: one network slept somewhat less deeply and reacted more strongly to novelty during the first laboratory night.
The headline is larger than the evidence
The study did not show that the whole left hemisphere stayed awake. It did not establish that every person uses the same side, that the effect lasts all night, or that an instrumented laboratory reproduces an ordinary hotel.
The participants were young and healthy, nearly all right-handed, and the separate experiments used subsets of the 35-person sample. Questions remain about children, older adults, left-handed people, frequent travelers, and people with sleep disorders.
Later evidence has made the mechanism less certain. A 2024 study found first-night-type changes across nonconsecutive nights, including recordings at home, but did not find that hemispheric slow-wave differences tracked the effect as the night-watch account would predict. That does not erase the earlier observation. It suggests the first-night effect is not one mechanism with one signature.
An unfamiliar soundtrack is harder to ignore
The sleeping brain continues to filter sensory information. It cannot wake for every refrigerator hum or distant car. Familiar sounds are easier to classify: at home, the brain has a history with the pipes, the boiler, and the neighbor’s midnight furniture rearrangement.
One interpretation is that unfamiliar sounds arrive with less learned context and are more likely to prompt checking. That is a plausible explanation for disturbed sleep, not a measurement of the cause in every traveler. Anticipation, stress, laboratory equipment, and disrupted routines are additional reasons a first night can differ.
For most people, adaptation is quick. Keeping a regular schedule, reducing obvious light and noise, setting a comfortable temperature, and avoiding repeated clock checks removes avoidable reasons to stay alert. Caffeine can complicate the night too; it can mute sleepiness without repaying sleep debt.
One restless night in a new place is therefore not proof that sleep has broken. It may be a temporary mix of unfamiliar surroundings and a filtering system that has not yet learned what it can safely ignore. Persistent poor sleep or regular daytime impairment is a different problem and deserves proper assessment rather than being dismissed as a first-night effect.
Visual guide
How the pieces fit together
