What was the snack, Teresa? It may be that it wasn't quite big enough, or your body dealt with it too quickly, so you had a liver dump during the night to raise your levels to your fasting reading.
That's just a guess, mind - I'm not on insulin, so I could be wrong. I'd be okay with 4.1 at bedtime, and if I had anything then it might be a couple of oatcakes with butter and some cheese - the fat and protein last longer than carb.
I hope someone with more experience will be along soon to help.
Viv 8)