You set yourself back
As stated, when you sleep, skin regenerates and heals itself. One night of sleeping with makeup on can and will disrupt the delicate balance of your skin and set you back. When you sleep, your skin regenerates. When your pores are blocked, your skin can't breathe, and thus it can't regenerate. In other words you are stifling your pores. The oil in your pores build-up and transform itself into a bacteria, which can lead to various skin problems. After a night with your makeup on, your skin will need about a week to restore its previous condition and to balance again everything. A whole week? It's not worth it. Take that makeup off, people!
You speed up the aging process (a.k.a you get wrinkles faster)
Sleeping in makeup clogs pores causing breakouts, dries out the skin causing redness and sensitivity, and speeds up the aging process leading to premature wrinkles.Not removing makeup before you go to bed means that it cakes onto your face. As wonderful as makeup is, if it is not properly removed before bed, it can lead to a collagen breakdown. If collagen cannot be produced properly, it means your skin ages faster than it is supposed to; drying out, it creates more wrinkles and helps to deepen existing ones. I don't know of anyone who likes to look twice their age.
Your skin dries out
Leaving makeup on overnight, even if you wash your face in the morning, causes flakiness, unevenness, and dryness, etc. If you properly cleanse your face at night, and moisturize with products that include ingredients like aloe or witch hazel, this gives your skin chance to repair and replenish itself at night. Not removing your makeup will deny your skin this chance and instead cause it to dry out. As a result, every time you wrinkle your forehead or squint in your sleep, the impression will stay there for longer.
You end up with an uneven skintone
Skipping out on proper skincare can also make an already uneven skin tone worse. This is because your skin regenerates at night as you sleep, so depriving it of ingredients known to combat pigmentation means that any recourse for skin damage has been removed. The same principal applies to age spots/dry spots too.
A simple selfcare can eliminate weeks and months of headache trying to fix your skin. Selfcare, practice it. Start by cleaning makeup from your face.