Preliminary Results: The Impact of Smartphone Use and Short-Wavelength Light during the Evening on Circadian Rhythm, Sleep and Alertness.
Level 3 - non-randomized controlled study
Within-subject laboratory crossover trial without explicit mention of randomization in the abstract
PubMed 33499010 · doi:10.3390/clockssleep3010005
What was done
Fourteen young male participants (mean age 21.93 ± 2.17 years) completed a laboratory sleep study across three separate nights. Before bedtime, participants spent 90 minutes reading under three different conditions: (1) on a smartphone with a blue light filter, (2) on a smartphone without a filter, and (3) on printed material. The authors assessed subjective sleepiness, objective alertness via a GO/NOGO task in the evening and morning, evening melatonin secretion, nocturnal vasodilation (distal-proximal skin temperature gradient), polysomnographic sleep parameters (early slow-wave sleep and slow-wave activity), and morning cortisol response.
What was found
Reading on a smartphone decreased subjective evening sleepiness compared to printed material, though it showed no effect on evening objective alertness on the GO/NOGO task. Evening melatonin levels and nocturnal vasodilation were higher following reading on printed material. Morning cortisol was elevated after reading on an unfiltered smartphone, resulting in a reduced cortisol awakening response. Early slow-wave sleep/activity and next-morning objective alertness were reduced specifically after using the smartphone without a blue light filter. The abstract did not report numerical values, effect sizes, or p-values.
Why it matters
This study provides mechanistic evidence that evening short-wavelength light from screens impairs not only evening sleepiness and melatonin secretion but also physiological sleep depth and next-day cognitive alertness. It suggests that blue light filters can partially attenuate these carryover effects.
Limits
The study is based on a very small preliminary sample (n = 14) restricted entirely to young adult men, limiting generalizability. Randomization and counterbalancing sequences are not detailed in the abstract. Real-world behavioral factors such as screen content, interactive engagement, and habitual sleep patterns were not evaluated under this controlled reading protocol.
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