You say you're EC and your DS's are 13 months, I'm thinking the problem maybe as you said in your original post and that it's due to greater awareness at night. We have used EC with my DD and she would wake at night, I suspect when she wee'd. She is however at 15 months now dry at night
though we still pop a night nappy on due to the odd night she'll wee, usually I suspect in the early hours of the morning when she has a feed, or nights where she has extra water or milk.
I'm not sure how easy it is to tell in a sposie, not having used them since DD was about 2 months old, but can you tell how wet they are in the morning if they haven't woken due to being wet? I'm just thinking if they're very dry, DD was probably having just one wee a night by 13 months, I wouldn't bother buying special night nappies. We just put DD in a normal day nappy and don't even bother with a cover. That way I can tell when she has wet and get her to the toilet. She goes a good 12 or 13 hours without using the toilet, or occasionally gets up between bedtime and midnight and 'asks' to use the loo before going back to sleep.