There is no known formation in the handwriting which can discriminate an early riser from the late riser.
Looking at reasons why someone might tend to sleep till late, we can identify those traits in the handwriting.
1. A heavy pressure writer needs long deep sleep, as his energy level is high he needs to relax more.
2. A person with stretched a and o in the writing prefers comfort over achievement, so s/he might sleep in day time too.
3. A person with a wavy baseline does not have a reason to get up as he is not focused on his goals, so he might just keep sleeping.
If you can share the handwriting of the person you are concerned about the exact reason can be found out.
Handwriting Analysis is not rocket science but some Graphologists make tall claims and then it all goes towards divinition and forcasting magically.