It depends why you're not eating. Is it because you're not hungry/don't feel like it, a sensory thing about the texture of food, finance issues, home issues, feeling like you don't deserve it, etc? There are a lot of factors that can contribute to that, and it's important to identify that to work out how to improve it.
It's normal (not necessarily good, but normal) to feel weak after not eating - your body goes into what's called the post-absorptive state about 3 hours after your last meal, at which point it's having to use your body's fuel stores to generate energy to keep you alive - even just resting and stay alive requires about 1000 calories a day, and that's not taking into account any exercise whatsoever. So if it's been a significant number of hours since you last ate, you'd be using up those fuel stores, which makes you feel weak because your body is having to do more work breaking those stores down to keep you alive and so there's not much energy available to keep you going about your day as being alive is, from a survival standpoint, more important than having energy to do stuff.
So I'd definitely recommend identifying why it is that you're choosing sleep instead of food, and maybe try and implement something to combat that, whether it's having a snack before you go to sleep or talking to someone if there's an underlying issue going on.