This usually happens right after a misting. Mine will be reading 54 or so by the time I go to bed, but if I don't mist it, I will wake up and find it at 40. So I always mist before bed, and it usually rises quickly to around 82. I have waited up to see how long it takes to come down, and it's usually less than hour before it balances out around 62. From there is seems to creep all the way down to 48-50 overnight so I mist lightly in the morning before I leave for work. This causes another, smaller, spike.
Sadly, I am coming to realize that this is the major downside to glass enclosures, humidity doesn't hold well. Even with the top mostly covered and a wet sponge under the lamp, etc.
My plan is to convert to a PVC enclosure with a radiant heat panel. This should help reduce the amount of meddling I have to do to keep the humidity in line. The way I see it, the less I have to mess with the enclosure everyday, the happier my snake will be.