Not a really serious problem now but could be leading up to one.
Every game is slightly different so you have to find the cause. I would first look at building is there enough housing or too much building going on.
Are your farms producing enough grain fast enough to allow villagers time enough to sleep. It s all about the art of designing correctly unemployed villagers should be kept to a minimum by creating jobs and not by making disciples.
A quick example you start a new land and you find the most fertile land is far to the west build out to near the center with roads and villas, now place a windmill reasonably close to the center and surround it with minimum sized fields can feed 10,000 but badly placed fields can struggle to feed 100 for the same area.
its a balance problem, when you keep in one land foran large period of time, the value increase too fast, fast enought to grow your villager sleep desiere and free time desiere in seconds, i fixed this problem with a mod, see my mod thread (migx mods (download)) check the mod/fix,happy building
I dont think this is the problem, in fact its the opposite. A single night of sleep can supply many days of nightless work.
I know because i dont like night because i cant see anything and they dont work so im always at day time. After 2 hours the sleep need is full but a couple of complete nights solves this.
Uhm yup, as i said, the problem is the ammount, exponential bro, the sleep and free time gets glitchy when much time is played on the same land, so, the only way to fix this is deleting both desieres...
I keep in the norse land building great cities... And after 8 - 9 hours of gameplay (even not continously) the only thing the villagers want to do is sleep, even if u set nightime for hours.