I know that there is loss of muscle tone during REM sleep. Is this loss in muscle tone the reason I generally drop my pen in a sleepy lecture? But we were taught that sleep has 4 stages and REM sleep occurs after the 4 stages, ~90mins(?) post sleep. So technically, it should be that I'd be dropping my pen after 90mins of sleep. Though this is not the case. The pen drops generally when I'm at verge of sleeping.
I realise that there also is muscle atonia during other stages of sleep but, there is this famous story of Salvador Dali and the way he slept. He had a key in his hand and kept a tin plate beneath, as soon as he reached REm (dream) sleep, the key would fall and he'd wake up to draw stuff that he saw in his dream.
Why did Dali's key drop at REM sleep but my pen, way before that?