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.
