Waiting - SXEmacs Lisp Reference Manual

The wait functions are designed to wait for a certain amount of time to pass or until there is input. For example, you may wish to pause in the middle of a computation to allow the user time to view the display. sit-for pauses and updates the screen, and returns immediately if input comes in, while sleep-for pauses without updating the screen.

Note: In FSF Emacs the commands sit-for and sleep-for take two arguments to specify the time (one integer and one float value), instead of a single argument that can be either an integer or a float.

— Function: sit-for seconds &optional nodisplay

This function performs redisplay (provided there is no pending input from the user), then waits seconds seconds, or until input is available. The result is t if sit-for waited the full time with no input arriving (see input-pending-p in Peeking and Discarding). Otherwise, the value is nil.
The argument seconds need not be an integer. If it is a floating point number, sit-for waits for a fractional number of seconds.
Redisplay is normally preempted if input arrives, and does not happen at all if input is available before it starts. (You can force screen updating in such a case by using force-redisplay. See Refresh Screen.) If there is no input pending, you can force an update with no delay by using (sit-for 0).
If nodisplay is non-nil, then sit-for does not redisplay, but it still returns as soon as input is available (or when the timeout elapses).
The usual purpose of sit-for is to give the user time to read text that you display.

— Function: sleep-for seconds

This function simply pauses for seconds seconds without updating the display. This function pays no attention to available input. It returns nil.
The argument seconds need not be an integer. If it is a floating point number, sleep-for waits for a fractional number of seconds.
Use sleep-for when you wish to guarantee a delay.

25.7 Waiting for Elapsed Time or Input