I'm exhausted. Is it acceptable to sleep at 18:40?

As an adult it is acceptable to do whatever you want. You must be prepared for the consequences, however.

As long as it's acceptable to be awake at 0200.

Yup

Yep

Ye... 😴

Go right ahead.

Sure! But be warned, you may wake up at stupid o'clock in the morning tomorrow because you've slept all your body needs to, and perpetuate the cycle if you're not careful.