system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we close our eyes when we sleep? Also, why can't we fall half asleep by closing one eye for extended periods like dolphins do?
gpt
A few things:

1. we can sleep with one eye open, technically...just like we can sleep with both open.  

2. We don't have the capacity to put a single hemisphere of our brain into "sleep mode".  This is the wonderful capability of the dolphin and it's not so much their eye (although that part is true) it's their _brain_.  Our brains can't do this - we're all or nothing in the sleep department.