How Smart Is It to Go to Bed with the Phone? The Impact of Short-Wavelength Light and Affective States on Sleep and Circadian Rhythms.
Level 2 - randomized trial
Controlled within-subject crossover experimental trial in humans
PubMed 34842631 · doi:10.3390/clockssleep3040040
What was done
In a laboratory crossover trial, 33 men were evaluated across three experimental nights using polysomnography and body temperature monitoring. Prior to sleep, participants read for 90 minutes across three conditions: on a smartphone without a blue-light filter, on a smartphone with a blue-light filter, or from a book. Cortisol, melatonin, and affectivity were measured before and after sleep.
What was found
The abstract reports directional outcomes without exact numerical values. Reading on a smartphone without a filter reduced slow-wave sleep and slow-wave activity in the first quarter of the night and reduced the cortisol-awakening response. Melatonin increase in the evening was attenuated in both smartphone conditions compared to reading a book. The distal-proximal skin temperature gradient increased less following short-wavelength light exposure compared to reading a book. Subjective sleepiness did not differ, and higher evening positive affect predicted better subjective sleep quality without altering objective sleep measures.
Why it matters
This study demonstrates that pre-sleep short-wavelength smartphone light directly alters slow-wave sleep architecture, skin temperature dynamics, and circadian hormone patterns, with blue-light filters offering only partial mitigation.
Limits
The study included a small sample restricted entirely to men (n = 33), limiting generalizability to females and broader demographics. No exact numerical data, effect sizes, or confidence intervals were provided in the abstract.