Is an Enderman classified as a monster?

I'm trying to go to bed, but it's not letting me. I have a showcase in my house that is upstairs, and that showcase has a trapped Enderman in it. Everything else in my showcase is an animal. Could it be that the Enderman that is causing my bed to tell me I can't sleep?

According to the MC wiki, a hostile mob prevents you sleeping. An enderman is considered a neutral mob unless provoked, so it should not prevent you sleeping.

In recent and current versions of Minecraft, nightmares are gone, but you cannot sleep if a hostile mob is within 8 blocks of the bed horizontally (along each axis) and 5 blocks vertically.

also

An Enderman (plural Endermen) is a three-block-high, humanoid, neutral mob that will turn hostile when provoked.

To answer your question, an enderman is not considered a monster. Regardless of this I tested this in the latest version of Minecraft and it did prevent me sleeping. So although it's classified wrong, it still prevents you from sleeping.