During sleep, our sense organs may still receive stimuli from our immediate environment. Our skin feels cold, our ears hear sounds, or our eyes may see a flash of lightning if it is raining outdoors.
If you dream of hearing an alarm clock ringing, you may wake up. If your alarm clock really rang, it means you heard this stimulus while sleeping and it entered your brain as a dream of an alarm clock.
If you dream of an alarm clock, wake up and no physical alarm clock was ringing, it means your brain probably was used to wake up at a certain hour and just did what it was accustomed to do.
Prompt – alarm.