QVC.com sells xmas lights that run on batteries/timers... I have lots of them for my house, but now have a new purpose for them
I especially like that they have timers!

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They will sleep fine with a light on. They do during the day as well.I have a 13w CFL in my coop on 24/7. I was not aware of circadian rhythm until I read this post, but it makes sense to let them sleep a little. I attributed the light being on to them laying in the winter, but it might be just because they are happy birds.