Coincidence?!

I've read that sleep is important - that to be healthy, a person should sleep from seven to nine hours a night. 

But for years now, I've had trouble sleeping through the night. I go to bed when I start to feel tired, often around 9 pm.

Then, five or six hours later, I wake up.

I get up and read for a while. Sometimes after that, I'm tired enough to fall asleep again.

BUT... I've recently discovered that if the weather is nice, and I take a brief walk around the block in the evening, I  sleep through the night!!

Is that a coincidence?! Maybe I need to make it a regular habit... an evening walk!