Sleeping bag on road to athens?


Hi im doing the road to athens trip leaving 30th august!
the itinery says that a sleeping bag is required for the 3 nights sailing in greece. although i have heard that most people do not use them and just take an inner sheet. could anyone give me any advice as im travelling alot after this trip and would prefer not to have to take a sleeping bag for only 3 nights!! hopefully admin could also answer.