Smartphone usage strongly increased in the last decade, especially before bedtime. There is growing evidence that short-wavelength light affects hormonal secretion, thermoregulation, sleep and alertness. Whether blue light filters can attenuate these negative effects is still not clear. Therefore, here, we present preliminary data of 14 male participants (21.93 ± 2.17 years), who spent three nights in the sleep laboratory, reading 90 min either on a smartphone (1) with or (2) without a blue light filter, or (3) on printed material before bedtime. Subjective sleepiness was decreased during reading on a smartphone, but no effects were present on evening objective alertness in a GO/NOGO task. Cortisol was elevated in the morning after reading on the smartphone without a filter, which resulted in a reduced cortisol awakening response. Evening melatonin and nightly vasodilation (i.e., distal-proximal skin temperature gradient) were increased after reading on printed material. Early slow wave sleep/activity and objective alertness in the morning were only reduced after reading without a filter. These results indicate that short-wavelength light affects not only circadian rhythm and evening sleepiness but causes further effects on sleep physiology and alertness in the morning. Using a blue light filter in the evening partially reduces these negative effects.

Previously, we presented our preliminary results (N = 14) investigating the effects of short-wavelength light from a smartphone during the evening on sleep and circadian rhythms (Höhn et al., 2021). Here, we now demonstrate our full sample (N = 33 men), where polysomnography and body temperature were recorded during three experimental nights and subjects read for 90 min on a smartphone with or without a filter or from a book. Cortisol, melatonin and affectivity were assessed before and after sleep. These results confirm our earlier findings, indicating reduced slow-wave-sleep and -activity in the first night quarter after reading on the smartphone without a filter. The same was true for the cortisol-awakening-response. Although subjective sleepiness was not affected, the evening melatonin increase was attenuated in both smartphone conditions. Accordingly, the distal-proximal skin temperature gradient increased less after short-wavelength light exposure than after reading a book. Interestingly, we could unravel within this full dataset that higher positive affectivity in the evening predicted better subjective but not objective sleep quality. Our results show disruptive consequences of short-wavelength light for sleep and circadian rhythmicity with a partially attenuating effect of blue-light filters. Furthermore, affective states influence subjective sleep quality and should be considered, whenever investigating sleep and circadian rhythms.

Background: Healthy circadian rhythmicity has been suggested to relate to a better state of brain‐injured patients and to support the emergence of consciousness in patient groups characterized by a relative instability thereof such as patients with disorders of consciousness (DOC).Methods: Going beyond earlier studies, a systems‐level perspective was adopted and, using multilevel modelling, the joint predictive value of three indices of circadian rhythm integrity derived from skin temperature variations, melatoninsulfate secretion, and physical activity (wrist actigraphy) patterns was evaluated for the behaviourally assessed state [Coma Recovery Scale ‐ Revised (CRS‐R) score] of DOC patients [13 unresponsive wakefulness syndrome; seven minimally conscious (exit) state]. Additionally, it was assessed in a subset of 16 patients whether patients’ behavioural repertoire (CRS‐R score) varied (i) with time of day or (ii) offset from the body temperature maximum (BT_max), i.e. when cognitive performance is expected to peak.Results: The results reveal that better integrity of circadian melatoninsulfate and temperature rhythms relate to a richer behavioural repertoire. Moreover, higher CRS‐R scores are, by trend, related to assessments taking place at a later daytime or deviating less from the pre‐specified time of occurrence of BT_max.Conclusions: In conclusion, the results suggest that therapeutic approaches aimed at improving circadian rhythms in brain‐injured patients are promising and should be implemented in hospitals or nursing homes. Beyond this, it might be helpful to schedule diagnostic procedures and therapies around the (pre‐assessed) BT_max (≈4 pm in healthy individuals) as this is when patients should be most responsive.

While light therapy has proven effective in re-entraining circadian rhythms, the potential of such an intervention has not been evaluated systematically in post-comatose patients with disorders of consciousness (DOC), who often have strongly altered circadian rhythms. We recorded skin temperature over 7–8 days in patients with DOC in each of two conditions: habitual light (HL), and dynamic daylight (DDL) condition. While patients were in a room with usual clinic lighting in the HL condition, they were in an otherwise comparable room with biodynamic lighting (i.e. higher illuminance and dynamic changes in spectral characteristics during the day) in the DDL condition. To detect rhythmicity in the patients’ temperature data, we computed Lomb–Scargle periodograms and analyzed normalized power, and peak period. Furthermore, we computed interdaily stability and intradaily variability, which provide information about rhythm entrainment and fragmentation. We analyzed data from 17 patients with DOC (i.e. unresponsive wakefulness syndrome [n = 15] and minimally conscious state [n = 2]). The period length of the patients’ temperature rhythms was closer to 24 h in the DDL as compared to the HL condition (median median deviation from 24 h: DDL = 0.52 h, HL = 3.62 h). Specifically, in 11/17 (65%) patients the period length was closer to 24 h in the DDL condition. Furthermore, the patients’ rhythm was more pronounced, more stable, and less variable in the DDL condition. Our results indicate that DDL stimulation entrains and stabilizes circadian rhythms. This highlights the importance of adequate room lighting as an adjunct therapeutic approach for improving circadian rhythms in severely brain-injured patients. German Clinical Trials Register (DRKS00016041); registration: 18.01.2019; recording start: 04.06.2019 https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00016041

The current study investigated heart rate (HR) and heart rate variability (HRV) across day and night in patients with disorders of consciousness (DOC). We recorded 24-h electrocardiography in 26 patients with DOC (i.e., unresponsive wakefulness syndrome (UWS; n = 16) and (exit) minimally conscious state ((E)MCS; n = 10)). To examine diurnal variations, HR and HRV indices in the time, frequency, and entropy domains were computed for periods of clear day- (forenoon: 8 a.m.–2 p.m.; afternoon: 2 p.m.–8 p.m.) and nighttime (11 p.m.–5 a.m.). The results indicate that patients’ interbeat intervals (IBIs) were larger during the night than during the day, indicating HR slowing. The patients in UWS showed larger IBIs compared to the patients in (E)MCS, and the patients with non-traumatic brain injury showed lower HRV entropy than the patients with traumatic brain injury. Additionally, higher HRV entropy was associated with higher EEG entropy during the night. Thus, cardiac activity varies with a diurnal pattern in patients with DOC and can differentiate between patients’ diagnoses and etiologies. Moreover, the interaction of heart and brain appears to follow a diurnal rhythm. Thus, HR and HRV seem to mirror the integrity of brain functioning and, consequently, might serve as supplementary measures for improving the validity of assessments in patients with DOC.