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Bright light effects at night and day on various measures of sleepiness
Objective: Nighttime bright light suppresses melatonin and reduces sleepiness. This reduction in sleepiness is associated with a reduction in the theta power of the Wake-EEG. Exogenous melatonin administration increases not only subjective sleepiness but also the theta power in the wake-EEG. This suggests that the reduction in sleepiness is mediated by melatonin. To test this we compared the activating properties of bright light exposure during nighttime with those during daytime, i.e. in the absence of melatonin. Methods and Material: A total number of 24 subjects participated in two experiments (N=12 each). All subjects were non-smoking healthy young males (age, mean±SD: 22,6±1.9 years). In the first experiment subjects were exposed to either bright light (5000 lux) or dim light <10 lux (control condition) between midnight and 4 a.m. In the second experiment subjects were exposed to either bright light or dim light between 12 a.m. and 4 p.m. Hourly measurements included salivary melatonin and cortisol concentrations, 6 minutes ECG, sleepiness (Karolinska Sleepiness Scale), and wake-EEG (3min with eyes closed). Effects of bright light were tested by repeated measurements ANOVAs for the 3 hours prior to light exposure against the three hours during light exposure. The measurements at lights-on were not included in this analysis. Results: Sleepiness (KSS) was significantly reduced in the ocular light condition independent whether light exposure took place during nighttime (F (11,1)=11.29, p<0.01) or daytime (F (11,1)=9.61, p=0.01). There was a significant (F (11,1)=31.20, p<0.001) melatonin suppression during nighttime light exposure whereas no melatonin was present, and suppressed during daytime. Heartrate increased (F (11,1)=22.92, p=0.001) and the variability in interbeat interval (F (11,1)=6.27, p=0.029) decreased significantly during nighttime but not during daytime exposure. Both during nighttime and during daytime light exposure there was no overall effect on the cortisol levels. Analysis of nighttime eyes closed wake-EEG revealed an increase in the power density in the theta frequency band with increasing duration of wakefulness which was significantly correlated with the increase of subjective sleepiness over time (r=0.43, p<0.001). During bright light exposure at night power density in the theta frequencies is lower than during the dim light condition (p<0.05) and this decrease was significantly correlated with the decrease in subjective sleepiness (Rs=0.65, p<0.05). Daytime bright light exposure had no effect on the theta frequency of the wake-EEG. Conclusions: Nighttime reduction of sleepiness by bright light is accompanied by physiological changes in melatonin, heart rate, theta frequency power density but not in cortisol. Daytime bright light exposure influences only sleepiness without a change in any of these physiological variables, which suggests that the reduction of sleepiness is not necessarily mediated through the suppression of melatonin. Subjective sleepiness at night seems to be related to the theta frequency in cortical wake EEG activity.