Lack of sleep impairs the mind's ability to recall facts efficiently. Still in a dream-like state, the mind jumbles memory so that you may claim with high confidence to remember things that in fact never happened, typically due to strong semantic associations with actually encoded events, say the researchers at Luebeck University, Heinrich Heine University in Dusseldorf and the universities of Geneva and Zurich.

In other words, sleep deprivation means the mind has not finished sorting and storing memories. It is a bit like when your computer warns you that some unsaved files may be lost, if you reboot your computer without waiting for it to save everything first.