This may be useful for some. If you've managed to get your bedroom really dark and you do get up during the night, so that you don't fall over something you might use a Maglite AA or something of the sort. Now if you put a red filter over the lens, the wrong hormone cascade won't go off, as would happen if you used white light.

This little kit has the red filter in:

Amazon.com: MAGLITE AM2A016 Mini AA Flashlight Accessory Pack: Home Improvement

Mini Maglite 2-Cell AA Accessory Pack: Amazon.co.uk: Lighting

Otherwise, I guess you could stick some red cellophane over the lens.