Surely we need sunlight to sleep properly. But how much? Rats can be synchronized to a 24-hour activity rhythm with a relatively small amount of light (such as one hour) every 24 hours. This is one reason for the emphasis on morning light by sleep doctors mentioned in a previous post.
I have agreed with them. For the last 10 years I have gotten one hour of sunlight-like light every morning from a bank of fluorescent lights on the handles of my treadmill. The lights shined up at me while I exercised and watched TV. This, I thought, allowed me to get a good dose of light with low variance in when and how much and to combine light-getting with exercise. I never questioned this routine.
Then came the event that led to this Sunlight and Sleep series: In the airport during a trip to New Orleans, a student told me when she sunbathes, she sleeps better. When I got home from my trip I tested her idea. Me, too: When I was outdoors a lot (in the shade), I slept better.
I took another trip (to Los Angeles). When I got back from that trip, I decided that I would adjust the timing of the treadmill light so that it interfered less with my day. I shifted it from 9:00 am to 10:00 am (original timing) to 8:00 am to 9:00 am (new timing).
To my surprise I started waking up too early, so often it could not be a coincidence. The only change I had made was timing of the light. So the treadmill light was making things worse! I stopped it entirely. My sleep improved — no more early awakening. Huh.
Here are details:
Period 1 (treadmill light 9-10 am, little sunlight): woke up early 29 days out of 99 (29%)
Period 2 (treadmill light 9-10 am, lots of sunlight): woke up early 1 day out of 25 (4%)
Period 3 (treadmill light 8-9 am, lots of sunlight): woke up early 4 days out 8 (50%)
Period 4 (no treadmill light, lots of sunlight): woke up early 0 days out of 8 (0%).
Lots of sunlight means 6-8 hours exposure to light of roughly 1000-2000 lux. Sitting in the shade or inside next to a big window is always enough. At the low end (1000 lux) my laptop screen is easy to read; at the high end (2000 lux), which I try to avoid, it becomes slightly hard to read.