Exposure to smartphone light suppresses melatonin levels at night

A recent study published in the journal Brain Communications found that reading on a smartphone without a blue-light filter significantly reduced melatonin levels, especially in adults, compared to reading with a blue-light filter or from a printed book. However, while adolescents were able to recover their melatonin levels before bedtime, adults continued to experience reduced melatonin levels. This suggests that avoiding smartphone use in the last hour before bedtime is advisable to prevent sleep disturbances.

With the increasing use of smartphones and other devices emitting artificial short-wavelength light, there are growing concerns about their effects on sleep. These devices often disrupt the body’s natural melatonin production. The new study was designed to explore whether blue-light filters could mitigate these effects and to compare the responses of adolescents and adults to evening light exposure.

Melatonin is a hormone produced by the pineal gland in the brain that regulates sleep-wake cycles. Its production is influenced by light exposure: levels rise in the evening as it gets dark, promoting sleepiness, and fall in the morning with exposure to light, helping to wake the body. Melatonin is crucial for synchronizing the body’s internal clock with the natural day-night cycle, ensuring restful and consistent sleep.

“Using our smartphones and other light-emitting screen-based devices during the late evening hours is becoming more and more common, not only in adolescents but also in adults,” said study author Christopher Höhn of the Laboratory of Sleep, Cognition and Consciousness at the University of Salzburg.

However, little has been known yet regarding developmental differences in light sensitivity and regarding the required recovery time to restore melatonin levels after short-wavelength light exposure. Furthermore, the usefulness of blue-light filtering software is also not entirely clear yet. For me, these were important things to consider and clarify as they affect most people’s everyday life.”

The study involved 68 male participants, split into two age groups: 33 adolescents (14-17 years old) and 35 young adults (18-25 years old). The participants were monitored over a period of 14 days. Each participant spent three nights in a sleep laboratory under different reading conditions: reading on a smartphone without a blue-light filter, with a blue-light filter, and from a printed book.

Polysomnography, a comprehensive sleep recording method, was used to monitor the participants’ sleep. Additionally, melatonin levels were measured using saliva samples collected at multiple points throughout the evening and before bedtime. The participants also rated their subjective sleepiness using a standardized scale. The researchers ensured that the participants maintained regular sleep schedules and avoided confounding factors such as medication, smoking, and irregular sleep patterns.

The researchers found that reading on a smartphone without a blue-light filter significantly suppressed melatonin levels in both adolescents and young adults. However, adolescents showed a faster recovery in melatonin levels before bedtime, whereas adults still exhibited significantly reduced melatonin levels at bedtime. This melatonin suppression effect was less pronounced when participants used a blue-light filter or read from a printed book.

“We did not expect that adolescents would recover more quickly from the light-induced melatonin suppression since it has been argued before that younger individuals are more sensitive to short-wavelength light,” Höhn told PsyPost. “Most likely, the higher secretion rates of melatonin in the adolescent sample can explain their quicker recovery and, therefore, our results do not necessarily contradict the assumption of higher light-sensitivity in general.”

Despite the reduced melatonin levels, subjective sleepiness ratings did not significantly differ between the different reading conditions. Both age groups reported increased sleepiness over the course of the evening, regardless of the light exposure. However, adolescents felt sleepier than adults at bedtime.

Regarding sleep architecture, or the structure of different sleep stages, the study found no significant effects from the different reading conditions on overall sleep quality. However, adults showed a slight reduction in deep sleep (N3 sleep) during the first part of the night after reading on a smartphone without a blue-light filter. This effect was not observed in adolescents.

“I think that the main message is that using your smartphone or being exposed to bright short-wavelength light in general should be avoided as much as possible in the last hour before bedtime,” Höhn explained. “We did not observe strong effects on sleep, but it has to be kept in mind that we ended our light exposure roughly 50 minutes before bedtime and still observed some effects on the subsequent sleep episode.

“Thus, one should not treat the lack of severe sleep-disrupting effects in our study as evidence for ‘no harm of evening short-wavelength light’ as we do not know how much more severe the effects would have been, if the participants used their smartphone until a few minutes before bedtime.”

The study, like all research, has some caveats. Firstly, it only included male participants to avoid potential sex differences in sleep and light sensitivity. This selective sample limits the generalizability of the findings to the broader population.

Secondly, the controlled reading conditions in the study may not fully reflect real-life smartphone use, which often involves more engaging and potentially stimulating activities. Lastly, the study’s design included a 50-minute break between the end of the reading session and bedtime, which is longer than the typical break before sleep in real-life scenarios.

“I would like to stress that our smartphone exposure only comprised reading stories and that it engaging in different activities (e.g., social media or gaming) on the device might elicit stronger effects that add on the alerting light effects,” Höhn added.

The study, “Effects of evening smartphone use on sleep and declarative memory consolidation in male adolescents and young adults,” was authored by Christopher Höhn, Michael A Hahn, Georg Gruber, Belinda Pletzer, Christian Cajochen, and Kerstin Hoedlmoser.