We forget in the coldest days of the winter season that we are not getting enough vitamin D, as we become depleted of it, we are not getting enough essential nutrients from what we have when the days are sunny. It is recommended by doctors that we get outside for at least 15 minutes a day because too much sun can cause healthy problems, ex skin cancer.
We then wonder how the sun is going to affect our facial hair but you just need to find the right balance of being in the sun because you do not want to cause any damage to your beard if you do not want to shave it all off.
If you're having troubles with the growth of your beard and you think that it is not growing at the speed it should be. You might want to consider going outside for 10-15mins everyday because there is a strong link between the sun exposure. With the sun giving off vitamin D and having the right amount it is helping product healthy levels of testosterone which is than converted into DHT ( A hormone that promotes beard growth.)
Remember getting Vitamin D whether it is from outside or even taking Vitamin D pills if it is not a sunny day. Will help keep your beard growing and looking healthy.