Ender dragons do not lie down in the bed while sleeping. Similar to armor stands, their main hitbox is still decreased. This issue relates to MC-118058.

Reproduction steps

1. /time set midnight

2. Position a bed with its foot at coordinates 0 -60 18

3. /gamerule mobGriefing false

4. /summon minecraft:ender_dragon ~ ~ ~ {SleepingX:-1,SleepingY:-60,SleepingZ:18}

   (You have the flexibility to modify the command coordinates if you've positioned the bed elsewhere.)

5. Press F3+B

Observed: The ender dragon does not lie down but retains a smaller main hitbox. (MC-148559 and MC-10919) Sleeping entities are typically rotated by 90°, but this behavior does not apply to the ender dragon in this case.
Expected: The ender dragon would lie down on the bed as well.

I abide by Minecraft's community standards and prohibit the transfer of ownership of this bug report to others. I intend to retain possession of the bug report at all times. Additionally, I should not be banned because I will continue to find more bugs and report them. Being banned would prevent me from reporting them.