You may be interested in this link between the short days at this time of year and one of the nastiest pests to afflict bees - the varroa mite.
The queen bee lays eggs almost non-stop throughout the year. But at this time of year - end of December / early Jan when the hive is normally completely dormant the queen lays no eggs. The varroa mite is very vulnerable at this time as the mite (which feeds on bees while they are developing in their cell after it has been closed over and sealed) has nowhere to hide. They will be riding around on the backs of adult bees in the hive and cannot hide away in the capped brrod cells as they can during the rest of the year.
So at this time the beekeeper can use a natural acid called oxalic acid to treat the bees against varroa. Oxalic acid is the same acid as is found in rhubarb leaves. When a weak solution of this acid is mixed with sugar and dripped onto the bees at this time of year it irritates the mites, they annoy the bees and the bees respond by grooming each other to remove the varroa mites - many fall off and if the hive has a mesh floor - as all should these days - then the mites fall though onto the ground below and die. This reduces the winter level of the mites and their numbers are so weakened that they become much less of a problem the following summer.
Varroa mites are found in almost all bee colonies and when the colony is growing strongly and all is well the bees can keep on top of their effects. However if the weather is poor or the colony encounters some set backs the varroa mite numbers can increase sharply and cause great problems and often wipe out the whole hive. The bees become weakened and the hatching bees can succumb to a number of varroa transmitted viruses which can do things such as restrict the newly hatched bee's wing size or affect their strength preventing them from emerging from their cell.
So that is why you will see me out with my oxalic acid bottle in a bee suit in the middle of winter!