system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes us to lose consciousness when we go to sleep?
gpt
We don't.  You're not unconscious when you're asleep.  Your level of consciousness has just being altered.  Unconscious means you're not able to react to any stimuli.  A sleeping person will react to stimuli that is strong enough, like an alarm clock.  An unconscious person won't wake up just because an alarm clock is going off otherwise that's how they'd wake up people from a coma.