During an ordinary night's sleep, the usual sequence of sleep stages is:

A) 1-2-3-4-REM-1-2-3-4...
B) 1-2-3-4-3-2-REM...
C) 1-2-3-4-REM-4-3-2-1...
D) 1-2-1-3-1-4-REM...

The correct sequence of sleep stages during an ordinary night's sleep is 1-2-3-4-3-2-REM, with each sleep stage being held for varying amounts of time throughout the night. This means that sleep begins with stage 1, progresses to stage 2, then 3 and 4, before cycling back through stages 3 and 2, and then on to the REM stage. This cycle repeats throughout the night, with longer periods of REM sleep occurring later in the night.