I think you can only sleep in the evening or at night. If it's morning or midday, you can't. How you tell if it's evening, I don't know.
<img src="/ubbthreads/images/graemlins/ohh.gif" alt="" /> If thats the case, then it should be really easy to tell when its day or night. <img src="/ubbthreads/images/graemlins/up.gif" alt="" /> Don't know what else to tell you but the obvious: If its bright outside, its daytime. <img src="/ubbthreads/images/graemlins/idea.gif" alt="" /> If its darker and you can see a light radius around your character, then its nighttime. <img src="/ubbthreads/images/graemlins/stupid.gif" alt="" />
If you can't notice this difference <img src="/ubbthreads/images/graemlins/think.gif" alt="" />, perhaps your gamma is set too high or too low? <img src="/ubbthreads/images/graemlins/exclamation.gif" alt="" /> Try adjusting the lighting and see if there's a difference. Hope that helps. <img src="/ubbthreads/images/graemlins/delight.gif" alt="" />