One more thing about the power of light over bedtime: Sometimes people do choose to shift their sleep schedules. Exposure to bright light can do this when applied before bedtime, or upon wakening. If you allow yourself to be in bright light before bed, you’ll fall asleep later, and you’ll wake up later. The later in the evening you are exposed to this bright light, the longer you will be delayed in falling asleep.
But about five hours after your usual bedtime (if your bedtime is usually at eleven p.m., this would be around four a.m.), your body temperature hits its lowest point. When your body is at this low temperature point, any bright light exposure from then until daybreak will cause you to wake up earlier and fall asleep sooner at the end of the day. Bright light does this effectively until about two hours after your regular wakeup time, at which point bright light has pretty much no effect on your sleep and wake cycle. That’s because your body is trained to be in bright light during the day. (Yes, I’m being Dr. Obvious here. Glad you caught that.)
Melatonin is continuously released throughout the night until the body perceives a gradual light increase, which the body can do through the eyelids. As darkness fades, so does melatonin production (usually about 1-2 hours before awakening)
Light and Your Circadian Rhythms
There are cells in your eyes called ipRGC’s (intrinsically photosensitive retinal ganglion cells) that control how light affects your circadian rhythms. The ipRGC cells respond most powerfully to blue light. Think about what color the sky is at dawn. Blue, right? How about that. These cells send signals to the pineal gland in the brain, informing it about how light it is outside.
Ever wake up and stand at the window, looking out at the dawn? And you sometimes just suddenly feel awake? This may be the melatonin in your system getting an abrupt shutdown from your exposure to light. Light can significantly reduce melatonin, and that’s why light can create such problems at night. Blue-lit screens such as those on phones, computers and TVs can wreak havoc on your circadian rhythms. They will send you the “it’s dawn!” signal even though it’s 10:30 at night, and you will, as a result, become less sleepy.