Smells affect dreams.

This may explain why my wife has terrible dreams the nights I eat dairy. From New Scientist:

Stuck's team waited until their subjects had entered the REM phase of sleep, the stage at which most dreams occur, and then exposed them to a high dose of smelly air for 10 seconds before waking them up one minute later. The volunteers were then quizzed about the content of their dreams and asked how it made them feel.

All subjects reported a positive dream experience when stimulated by the rose smell, and most experienced the opposite when exposed to the rotten eggs. Stuck says the smells influence the "emotional colouration" of the dream.

The team are now looking to recruit people who suffer from nightmares to see if exposure to smells can help make their dreams more pleasant.