I had exactly this problem when I first started wearing to bed - in my case with the Abena M4. I’m no expert but I have read many times in these threads that this is a very common problem most disposable diapers/nappies.
Moisture/liquid will always find the route of least resistance so it’s nearly inevitable that the weak spot will be at the sides where there is little/no padding, and gravity will always create a situation where it will run down, so urinating while lying on your back a certain amount will run to the sides. I now mainly use the plastic-backed Tena Active Fit Ultimas at night and have very few leaking problems, but the cloth-backed Abenas leaked from the sides most nights.
My solution is waterproof pants, either pvc/plastic or recently some much lighter plasticised nylon pants which work just as effectively. In my Abena days, I bought some thick ‘training pants’ which I wore under the plastic pants, but they were far too bulky for my liking and I’m not really into babyish patterns/designs.
With my present Tena nappies I wear with boxer briefs over the top anyway during the day, so at night I simply do the same with plastic pants over them. As I say, I hardly ever experience leaks with the Tenas, but on the rare occasions they do, the wetness is absorbed into the boxer briefs and contained within the waterproof pants.
For me, wet pants is easily preferable to a wet bed!
Off to a busy day now…
