Keeping your hands away from your face is a cardinal rule in staying healthy.
Considering that your hands and arms are touching things all day long, if you go to sleep without showering, or at least washing your arms, you'll be transferring germs from your arms to your pillow.

And... if you sleep with your arm under your head, you're now exposing your face to those germs and those germs will enter your system through your eyes and nose.

The simple solution is to take your daily shower just before going to sleep.  

Germs are everywhere.  The Corona virus is the most worrisome as of this writing

Here are some rules to live by.

RULE #1  KEEP YOUR HANDS AWAY FROM YOUR FACE.  
Many germs enter your system through your eyes and nose so rubbing your eyes and nose are not a good idea.  

It takes practice to avoid the itching but learn to do that for your own health.

RULE #2  COUGH and SNEEZE INTO YOUR ELBOW - NOT YOUR HAND.
People who cover the mouth and nose with their hands don't realize that every time they touch something they are spreading their germs.

Instead, sneeze into your elbow or upper arm.  Everyone will thank you. 

RULE #3.  DON'T SHAKE HANDS
Skin contact spreads germs quickly.  Don't do it.
Instead do a fist bump or elbow bump.  Recently bumping feet is becoming p0pular. 

Stay HEALTHY>