What it is
Sound machines play a steady sound: white noise (all frequencies equally, a hiss), pink or brown noise (weighted towards low frequencies, more like rushing water or a rumble), fan sounds or nature loops. The idea is masking: a constant background makes sudden changes such as traffic, doors or snoring less likely to wake you.
The features that matter are practical: sounds that don't audibly loop, fine volume control at the quiet end, and a timer.
How strong is the evidenceweak
A 2021 systematic review of 38 studies rated the evidence that continuous noise improves sleep as very low quality: results ranged from better sleep to worse, and the authors warned it may affect sleep and hearing. The widely reported 'pink noise boosts deep sleep and memory' findings come from lab systems that play brief sound pulses timed to brain waves, not from a machine playing all night.
The claims you’ll meet3 checked
Masks disruptive noise and helps you sleepMixedPlausible, and some people clearly find it helpful, but the trial evidence is very low quality and some studies found worse sleep.
Pink or brown noise deepens sleep and boosts memoryUnprovenLab studies used short pulses synchronised to slow brain waves, not continuous noise from a speaker.
Sound machines help babies sleepUnprovenLittle good evidence, and machines can be too loud close to a cot.
What to look for4
- ✓Non-looping sound and a choice of tones; many people prefer lower, 'brown' sounds.
- ✓A volume that goes genuinely low. You want just enough to mask, not a loud hiss.
- ✓A timer or fade-out if you only need help falling asleep.
- ✓A display that turns fully off, and mains power or a long battery life.
What to skip2
- ×'Clinically proven deep sleep' claims for continuous pink noise.
- ×Paying for dozens of sound tracks. Most people use one.
Take care2
- !For babies, place the machine well away from the cot, at low volume, ideally on a timer. In one test all 14 infant sound machines exceeded 50 dBA at 30 cm on maximum, and three exceeded 85 dBA.
- !For adults, use the lowest volume that works. Loud noise every night adds to your hearing load, and earplugs may do the job more gently.
The studies3 cited · checked against PubMed
Noise as a sleep aid: A systematic review ↗Riedy SM, Smith MG, Rocha S, Basner M · Sleep Medicine Reviews · 2021Systematic review
38 studies of continuous white or broadband noise. Findings ranged from improved to disrupted sleep, and by GRADE criteria the evidence that continuous noise improves sleep was very low quality; the authors caution it may harm sleep and hearing.
Infant sleep machines and hazardous sound pressure levels ↗Hugh SC, Wolter NE, Propst EJ, Gordon KA, Cushing SL, Papsin BC · Pediatrics · 2014Laboratory measurement study
14 infant sound machines at maximum volume all exceeded 50 dBA (the hospital nursery limit) at 30 cm, and three exceeded 85 dBA, a level that risks hearing damage if played for hours.
Auditory closed-loop stimulation of the sleep slow oscillation enhances memory ↗Ngo HV, Martinetz T, Born J, Mölle M · Neuron · 2013Randomized crossover study
In sleeping adults, brief sounds timed to the up-phase of slow brain waves enhanced slow oscillations, spindles and next-day memory; sounds played out of phase did nothing. This is timed stimulation, not continuous noise.