Kava Science Kava peak time and oversedation
I kind of understand your question but can't put a figure on it. What time are you starting to drink kava? I always start around 5.30 to 6 pm, up to about 7 at the latest usually, and will drink for quite a few hours with well spaced out shells, 45 mins to an hour. I have all my kavain fun early on and then later I feel more sedation than anything else. I should say I drink almost exclusively heavy kava.
I think maybe you are drinking a bit too close to bedtime to allow the heavier kl's to kick in? I find it better to start earlier and give the kava the time to show it's full range of effects before I try and sleep. If not then like you I will usually feel it more in the morning and the next day than at the time I really need the sedation. Saying that though if you're drinking heavy kava you're going to experience a fair amount of next day effects anyway depending on the dose and the kava.