I had read this thread a while ago, and just wanted to add my two-cents to the subject b/c my BF in med school just came home from his neuro class and refreshed my memory about the circadian rhythm...
While you sleep, the cones in your retina are still sensitive to blue wave lengths of light (this is because a majority of the light in the atmosphere is blue and humans used to just use the sun as a natural clock). A tiny cluster of cells in your brain called the suprachiasmatic nucleus is sensitive to these light signals from the retina and in response tell the ventrolateral preoptic nucleus (which is responsible for sleepiness) to stop firing and allow our arousal system to wake up again. Blue light (also a component in white light), will effectively reset your/your patient's circadian rhythm, which is probably why people have so much trouble sleeping in the hospital. Because the lights are constantly being turned on and off throughout the night, your patient's circadian rhythm is constantly being interrupted and reset. Red light would interrupt your patient's circadian rhythm least, but like HRM672 said, would make it difficult to evaluate perfusion issues.
Just some little pearls of knowledge!
There's also an article about the brain during sleep in the most recent National Geographic (take out the spaces):
http:// ngm.nationalgeographic. com/2010/05/sleep/max-text