What is "to sleep late"? [closed]

What does the expression "to sleep late" mean?

Does it mean " to sleep in" or "to go to bed late at night" or both?

Does it only mean "wake up late" or "sleep in"?

Yes.

Does it also mean "to go to bed late in the night"?

No.

You almost have the second meaning correct, but it's missing a word:

• to sleep late
  to sleep in or wake up late
  
  
• to go to sleep late
  to go to bed late at night

Sleep is a simple verb:

[Merriam-Webster]
1 : to rest in a state of sleep

But go to sleep is an idiom with a specific meaning:

[Merriam-Webster]
1 : to begin sleeping
   // She lay down on the couch and went (right) to sleep.
   // Tell the kids it's time to go to sleep.
   // After the party, I just wanted to go to sleep.

Note the use of begin in the definition rather than just the state itself.

It's the use of go that makes the specific difference in terms of the meaning.

According to The Free Dictionary:

sleep late: to sleep later than usual or customary; to sleep in; and

sleep in: to oversleep (e.g., I missed the morning train because I slept in.) or to sleep late on purpose (e.g., After this week's work, I will sleep in on Saturday.)

Neither of these has anything to do with "going to bed late at night."