Dr. I Sometimes Have A Problem Sleeping When Laying Down

Dr.
I sometimes have a problem sleeping when laying down at night in the last year or so especially on our sailboat. If I sit up it goes away. The last 3 nights I've experienced this problem at home and couldn't sleep till almost daybreak for about 3 hours or so. It's a weakness feeling and sometimes a chill and shakes. Does not bother me in the daytime. I'm 73 but in otherwise good health with workouts 3 times a week.