According to the National Sleep Foundation, exposure to light in the morning sends signals to the brain to increase body temperature and produce hormones like cortisol. Yes, increased cortisol levels are associated with higher levels of stress, but having higher cortisol levels in the morning is normal and helps us to wake up. Light exposure in the morning also delays the release of melatonin, which is associated with the onset of sleep and is produced in the brain when it’s dark.