Some smells can help you relax and to fall asleep. A lavender spray onto your pillow works wonders (avoid toilet de cow udder).
Sometimes a smell can wake you up. I was in a deep slumber when the unmistakable odour of skunk quaffed through the open window. The only thing to do was to close the window and spray an air freshener into the room. Another time, in the middle of winter with all the windows closed, I was awakened to the stench of death on a hot July afternoon. Then I remembered the double bean burritos I had for dinner (see skunk above).