Ever notice how there's a huge difference between waking up in the middle of the night, when it's nice and dark outside and you can easily fall back asleep, and waking up to sunlight streaming through your windows? Even if you pull the covers over your head, it can be really difficult to drift off again. And that's because — apart from it being bright — the brain reacts to sunlight in a way that basically screams for you to get out of bed.
"Sunlight in the morning affects your circadian rhythm, setting your body clock for the day and signaling you that it's time to wake up," Dr. Jeff Rodgers, DMD, D-ABDSM, D-ASBA, a sleep expert and dental sleep medicine practitioner, tells Bustle. This is likely to happen even if you're feeling tired, and even if you'd really like to continue sleeping in. For most of us, falling back asleep in the daylight just won't feel right.
"There is evidence to suggest that humans are diurnal creatures, meaning our circadian rhythms are set by the patterns of the sun," Rodgers says. "When you wake up due to sunlight, your body will naturally be alert." And as the day goes on, and the light gets dimmer, you'll naturally feel sleepy again.
To get even more science-y, "the retinas in our eyes have light-sensitive cells called photoreceptors that tell the brain whether it’s daytime or nighttime and thus affects our sleep cycle," Liz Brown, a certified sleep science coach and founder of SleepingLucid, tells Bustle. Exposure to sunlight, Brown says, can send signals to the brain that either says it's time to be awake.
That's why waking up to sunlight can also make it easier to get out of bed, if that's what you need to do. In fact, leaving the curtains open at night will help you wake up naturally. Some people even swear by sunrise alarm clocks, which are meant to ease you awake naturally by glowing like the morning sun on your bedside table, instead of ringing.
"Opening your eyes to a burst of bright sunlight can pull you out of your sleep grogginess before your alarm alerts you it's time to start your day," Dr. Kasey Nichols, a sleep expert at Rave Reviews, tells Bustle. And again, those photoreceptors in your eyes will do their thing by sending signals to the suprachiasmatic nucleus (SCN) and the occipital lobe in your brain, he says.
The SCN is your internal clock that responds to both light and melatonin levels. "Melatonin is a hormone released by the pineal gland that signals the SCN that it's time to go to sleep," Nichols says. "Melatonin is released in response to darkness (or the absence of light). So you are being woken up by two different mechanisms. Melatonin levels are decreasing due to the increased light exposure, and the SCN is being told that it's time to wake up by the presence of light."
If you want or need to keep sleeping, even when the sun is blazing, there are several quick fixes that can let your brain know it's OK to rest again. "It's possible to go back to sleep when woken up by the sun, but it will be harder to do so if the sunlight is still visible," Rodgers says. "You'll have to block it out."
One way is with a sleep mask, Rodgers says, especially if it's good quality and won't slip of your head at night, or let light creep in around the sides. Slap that across your eyes and you should be able to wake up when you want to, and not just because the sun is shining in. "Additionally, blackout curtains can help," he says, "particularly for night shift workers or even new [parents], who need to grab sleep outside of 'traditional' hours."
As humans, we're very tuned into the sun, and our internal clocks are set by it. For better or worse waking up to sunlight will signal to your brain that it's time to get up, and get the day going.
Dr. Jeff Rodgers, DMD, D-ABDSM, D-ASBA, sleep expert and dental sleep medicine practitioner
Liz Brown, certified sleep science coach and founder of SleepingLucid
Dr. Kasey Nichols, sleep expert at Rave Reviews