system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does one have no strength when waking up?

I don't normally talk like that, put if I replaced 'does one' with 'do I' my post will be removed for being too 'subjective'
gpt
Your body paralyzes itself while you're asleep so you won't act out your dreams. It's takes some time for your body to regain full strength from it after you wake up. 

Fun fact, this is the same cause as sleep paralysis, or waking nightmares. You wake up at night, but your body is still locked, so you can't move, and your still tired brain hallucinates nightmares.