Whether you realize it or not, over the course of the winter the air in your home gets progressively drier. It first came to my attention years ago when I found that by early March, touching one of my cats produced a small shock, and they were not too happy about that. While most plants would love the relative humidity to hover around 80%, that would make for a lousy home environment. So, they'll happily settle for 50%. But what most homes actually experience is closer to 10-15%.
And the amount of water in the air is also dependant on the temperature at which you keep your house- hence the term "relative humidity" Consider that if your thermostat is set to 27C, at 50% humidity the air is actually holding twice as much moisture as it would be if the temperature was 15C at the same 50%.
Central heating is great, but it dries the air and your bonsai suffer for it. As they dry out, it weakens the trees. Spider mites are my second least favorite result of dry winter air. My least favorite result? Dead trees and it can easily happen. One of my very favourite trees, the juniper procumbens nana a.k.a. Japanese garden juniper is particularly vulnerable to dry winter air. The same applies to Rosemary and in both cases, the reason that winter dryness is such a big problem is that neither tree will give you a lot of warning that they are struggling. Look at them and they appear to be fine. Green needles still in place and only an experienced eye and careful examination will alert you to the early stages of a problem. But you need to catch the problem early because once a tree hits that slippery downhill slope of failing health, it is very difficult to turn the tide. By the time you notice the problem it might be too late.
The first year I grew rosemary I thought that all was well. They looked fine, until I touched one and watched every needle on the tree fall off. It was a great teaching moment to understand that its entirely possible to think that your tree is fine when in fact it's already dead.
Junipers can be the same, looking just fine, thank you very much, until you realize that they have died, right in front of you and you didn't see it coming. It happens. Just for the record, anyone who grows bonsai will lose trees. There are bonsai that are easier to grow than others, and the easiest are labelled "beginner" but they really are advanced green thumb material (but worth the effort).
There are a few accepted remedies to dry winter air. On the high side there is that miracle of modern technology - the humidifier. Using cool mist, steam or whatever source, local or driven off the furnace, higher humidity will benefit all. While there are other commonly accepted alternatives to humidifiers, it really is the only effective option.
You can mist your plants and let's face it, a plastic spray bottle is a small investment, even to the most cash strapped amoung us. What you need to watch out for is overwatering. Yup, the water you spray on the foliage will usually drip to the soil and contribute to an overly soggy and potentially terminal root environment. So, by all means mist away, but direct the extra moisture away from the soil.
There is the pebble tray as well, and while its a great way to safely allow your pots to drain without trashing the furniture, its value as a humidity booster suffers from the same limitation as misting. Once the water evaporates it doesn't hang around the plant. It disperses into the air in every direction. Imagine how effective the evaporation of a cup of water is to raise the humidity in your living room (and I'm conveniently ignoring that most of the rooms in your home are likely not enclosed spaces).
So the tried and true combinations of misting and pebble trays are actually of very, very limited help. Sorry, but that's the truth. Still, as long as you don't over saturate the soil, they don't hurt and at least there is a tiny improvement. You could also consider briefly enclosing plants that seem to be suffering in plastic. For example this is actually an effective although temporary treatment against spider mites. I've over-nighted plants in large, hard plastic tote boxes set up like big pebble trays with a lid and removed the lid in the day. Sadly, they're not the most attractive additions to your home. I've used dry cleaners bags to create temporary greenhouses, but if you have children this is a terrible idea.
Luckily, while winter might feel endless, it isn't. Already by the start of February I can see that the sun is climbing higher in the sky and in the greenhouse the character of the light is changing for the better. Spring is coming. So get ready to throw open those windows at the first chance you get and move your trees outside for the summer. In the meantime, just do what you can. If you understand the nature of a potential problem, you're ahead of the game.