The study by the Rensselaer Polytechnic Institute, New York shows that morning light exposure may help to wake up the body when it is time to be active, thus preparing individuals for any environmental stress they might experience.
Levels of cortisol, a hormone produced by the adrenal gland, follow a daily 24-hour rhythm. Cortisol concentrations are low throughout the day, reaching a broad minimum in the evening before rising slowly again throughout the night.
In addition to this gradual elevation of cortisol at night, cortisol levels rise sharply within the first 30 to 60 minutes after waking.
This is known as the cortisol awakening response (CAR). In nocturnal animals, the cortisol spike occurs at night, at the start of activity.
It appears to be associated with the time of transition from rest to activity, upon waking. A high CAR has been associated with better preparedness for stressful and challenging activities.
"The present results are the first to show that low levels of short-wavelength light enhance CAR in adolescents who were restricted from sleep," said researcher Mariana Figueiro.
"Morning light exposure may help to wake up the body when it is time to be active, thus preparing individuals for any environmental stress they might experience," Figueiro said.
The study included three overnight sessions, at least one week apart. All participants wore a Dimesimeter on a wrist band to measure light exposure and to verify the regularity of their activity/rest periods during the three-week study.
The Dimesimeter is a small calibrated light meter device that continuously records circadian light and activity levels.
During the study, adolescents aged 12 to 17 years went to sleep at 1:30 am. And woke up at 6:00 am., a 4.5-hour sleep opportunity. Each week, participants either experienced morning short-wavelength blue light or remained in dim light.
"We found that exposure to short-wavelength blue light in the morning significantly enhances CAR in sleep-deprived adolescents, more so than dim light," said researcher Mark S Rea.
