The discovery of REM (rapid eye movement) and non-REM sleep has been important. Researchers have done many studies on this. Subjects have been woken up in both stages and asked what they were thinking about. It is clear that the reports from non-REM stages were different from REM stages. In particular, dreams occur mostly when the brain is in the REM state. There is also some relationship between dreaming and daydreams. Both seem to occur in a cycle of 90110 minutes.