There are 5 stages of sleep. REM sleep is the 5th stage and this is when dreaming occurs. We have at least 5-6 dreams a night and they start at about 5 minutes long and get progressively longer during each REM stage during the night. Towards morning a dream can last as long as 45 minutes or longer.
Some fun dream facts:
•People don't only dream in black and white, as was once believed.
•Even though they may not remember them, everyone dreams several times a night. In fact, during a typical lifetime, we spend about six years dreaming.
•People who have been blind from birth have dreams that are formed from their other senses (e.g., touch, smell, sound).
•When people are snoring, they're not dreaming.
•Elephants (and some other animals) sleep standing up during non-REM sleep, but lie down for REM sleep.