Everyone sleeps - it's universal, it's beyond our control, it's fascinating - so it's a good subject for writing.
Prime the pump by collecting references from Shakespeare. This is easy if you have the plays on a CD, but almost as easy on the internet. Try http:the-tech.mit. eduShakespeareworks.html
Give each member of your group a different play to search, looking for "sleep" and "dream", and copy-and-paste the results together for discussion.
Focus on the moments just before sleep, or on waking, or on dreams, and ask pupils to try to define and share the sensations.
The haiku is an excellent form for catching and expressing the ideas about sleep you have been discussing. Find guidance and examples at http:www.geocities.comhaiku_centralmain.html and then start writing.