I don't have your pituitary issues but slept on the sole for every
passage to SoPac and back. Wife got the the leeward berth. I used a camping mat and really had no issues with being comfortable. Actually, the motion of the
boat is less and easier to cope with down that low. Just be sure that the
boat you get has a deep sump to trap any
water that gets below. You don't want to sleep in sloshing
water which you will in a modern sumpless boat.
Still, you'll spend more time at
anchor so will want some berth that will fit you. Probably any boat will need to be modified if you want a berth you can stretch out in.