Screens in Bed: How Long is Too Long?

More than an hour of scrolling with the lights on nearly triples a student’s odds of poor sleep, and slows reaction time the next day

STUDY: Fallahmorad Z et al, Scientific Reports 2026 (Article in Press)

STUDY TYPE: Cross-sectional

FUNDING: Independent

Background

Smartphone screens emit blue light that suppresses melatonin, delays sleep onset, and disrupts the circadian rhythm. This study gives clearer metrics on usage, sleep quality, reaction time, and concentration the next day.

The Study

• 187 healthy Iranian university students (those with with depression, sleep disorders, shift work, or sleep medications were excluded)
• Smartphone usage was self-reported and measured in two conditions: lights on and lights off (not randomized)
• Sleep quality was assessed with the Pittsburgh Sleep Quality Index (PSQI)
• Concentration and attention were measured using a mobile app (U-Cancellation); reaction time was measured with a ruler drop test

Results

Students spent an average of 76 minutes on their phones before bed with the lights on, and 38 minutes with the lights off. Seven in ten (70%) scored in the poor sleep range on the PSQI.

Longer smartphone use before bed correlated with worse sleep quality and slower reaction times. The correlation was moderate for lights-on usage (r=0.49 for sleep quality; r=0.47 for reaction time) and weaker but still significant for lights-off usage.

In logistic regression, students who used their phones for more than 60 minutes with the lights on had 2.5 times higher odds of poor sleep. Those who used phones for more than 30 minutes with the lights off had 2.5 times higher odds as well.

Concentration and total cognitive performance were slightly lower in heavier phone users, though the correlations were weak (r=–0.19 for concentration; r=–0.15 for total performance).

Limitations

• Cross-sectional design: causation cannot be established. Self-report. No control group.
• Unmeasured confounders include anxiety, academic stress, chronotype, and bedroom environment.

Practice Implications

1. The study quality is limited, but the cut-offs are useful: 60 minutes with the lights on, or more than 30 minutes in the dark.
2. Before you write for a sleep med, ask about screen use.
3. Best to avoid the phone in bed completely, but if you have to use it, put some blue blocking glasses on.