Sleep tattoo refers to the markings on the body from sleeping for an extended period of time, caused by blankets, clothing, or any other thing one would sleep on. Commonly found on the chest, face, and arms. (Source: urbandictionary.com)

Sleep tattoo（睡眠纹身）指睡了很长一段时间醒来后，睡觉时盖的毯子、穿的衣服或者碰巧在床上的任何物品在身上留下的印记，多见于胸口、脸部以及胳膊。

For example:

Person 1: I just had the best nap of my life!

刚睡午觉起来，睡得可舒服了！

Person 2: What’s that all over your chest?

你胸口那儿怎么了？

Person 1: Oh those are just some sleep tattoos from my blanket.

哦，那是毯子留下的睡眠纹身。