When I’m really really tired and I just crash on my bed in the early evening, I usually wake up with lines on my arms and face from folds in the bed, but this never happens when I’m sleeping regularly.
When you are sleeping “regularly,” a assume you mean a lengthy slumber for the evening, whereas you get the lines from naps, am I correct?
If that’s the case, it’s simple… You move around in a deep slumber, which means that the time you spend laying on a particular crease or fold in the bed is lessened. Whereas during a nap, you do not fall into that deep REM sleep, you just lay there, and you get bed-head.