I was asked by one of my players today: If a simulacrum casts true polymorph on itself to turn itself into a monster that's not a construct, can it take rests and regain hit points? thoughts?

Simulacrum: The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.

True Polymorph: Creature into Creature: If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target's (or its level, if the target doesn't have a challenge rating). The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.

The target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious. 

From the Simulacrum spell's description, even though the Simulacrum is an illusion, anything that would affect a normal creature would affect the Simulacrum. So True Polymorph would work on it as long as the new form meets the criteria. The new form could rest and regain lost hit points, but those hit points do not get transferred to the original Simulacrum form. The Simulacrum form would have the same hit points it had when it was polymorphed, less any "spill over" if the new form was dropped due to damage. 

Yeah, it should be able to take rests