What's a sleeping mask and why you should add it to your skincare routine

I used to hate using masks - no matter if it was clay or sheet mask, I didn't enjoy it until I found a first mask that really worked for my skin - it was a mask from The Body Shop, back then I was poor student (I still am a poor student but back then I was a high school student) so I bought smaller versions of this mask and used it on pimples. Nowadays I love masks but lately, I started to use a sleeping mask more often than a sheet mask. What's a sleeping mask and why you should add it to your skincare routine?