Four hours. That's how long I slept until my dream ended. Well, I think it ended naturally because my alarm should have rang half an hour ago so there's nothing that could have woke me then. It's said intense dreaming occurred during REM sleep which happened after the four stages of NREM sleep. Each stage of NREM sleep typically took 5-15 minutes and REM sleep happened first around 90 minutes after start of sleep.
Since my dream ended after 4 hours, it should be my second REM sleep at least. I'm still unclear whether or not each NREM is immediately followed by REM but I think not, since REM started after 90 minutes and 4x15 minutes is less than 90.
Since the 3rd and 4th stage of NREM sleep are a period of deep sleep, I think waking up would be best during the 1st and 2nd stages of sleep. That means I have to set my alarm a little after one and half hour or four hours after I sleep when I sleep after midnight.
Anyway, I made the first part of this just after waking up yesterday. It just suddenly came to me because I didn't heard my alarm rang and I want to know when I slept the lightest. This is all because of the game I've been playing. I have to wake up more or less every two hours or my character would die.