decrease, diminish, lessen, fall - decrease in size, extent, or range; "The amount of homework decreased towards the end of the semester"; "The cabin pressure fell dramatically"; "her weight fall to under a hundred pounds"; "his voice fell to a whisper"

zonk out - fall asleep fast, as when one is extremely tired; "after the long drive, we zonked out and slept for 10 hours"

retrogress, regress, retrograde - get worse; fall back to a previous or worse condition

decline, worsen - grow worse; "Conditions in the slum worsened"

backslide, lapse - drop to a lower level, as in one's morals or standards