(This doesn't cover the usual stuff; if you're designing things for wide awake people, most normal guidelines work as long as you have proper illumination =)

The design of user interfaces for night time (as in "when people are trying to sleep") needs to take note of a couple of things:

In other words, it's pretty dark in the night and drowsy people are not good at operating things (even when they're simple to use in day), so make it simple.

Lack of senses: Usually, I find it good that remote controls arrange their buttons in some other formation than some standard grid - finding the correct buttons by memory and touch only is easy.

Also, some output forms that are useless on day are extremely useful in the night - my music player can say the name of the tune being played using speech synthetizer, which is handy because I can't read the screen.

Lowered consciousness and coordination: I always have problems with alarm clocks that are too hard to operate. The extreme case was the cellphone's alarm clock - before I noticed that it had a "snooze" feature, making it re-alarm later needed some serious button acrobatics that just don't work when I'd rather be sleeping.

Also, the controls need to be made big and simple. I think the Palm's alarm clock is cool: The alarm screen has two BIG buttons, OK and Snooze, and as long as I remember "It's the huge button on the right" I can stop worrying.

In conclusion: If you design things, make them simple. If you design things that people might use in the middle of the night without 6 cups of coffee, make them hyper-simple.

(And a disclaimer: I'm not an usability professional, this is just my own observation. =)

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