I have not slept on beaches, but I have, in parking lots(24 hrs Walgreens or Walmart) in some towns. I often check with the employees before doing so. Most of the times, they are helpful.
Unfortunately hwy1 does not have any rest areas. Else, it could be a good option.
Judge if it is safe & legal before you do anything.
And yeah, San Diego/ LA beaches might be comparitively warmer than the central coast beaches. I really doubt if it is wise to sleep in the open.
Have a safe trip,
cool