Personally I believe that as long as you get enough sleep for you (varries from about 6-9 hours per day) a set sleep schedule is not necessary for LDing. However, because a set sleep schedule can cut down on mild insomnia as well as make sure you are getting enough sleep on a regular basis it can be helpful.

I know that as long as I get enough sleep I can have as varried a sleep schedule as I need to and still LD.