It's that time of year when we yearn most for spring, but it's not quite here yet. Sunlight is low, and so is our energy and motivation.
When darkness falls earlier in the day, we start making melatonin earlier too, but we rarely go to bed when it gets dark in the winter - that could be so early! By the time we do go to bed, our melatonin may have worn off or we're busy making more, but increased melatonin can decrease serotonin. Then we get into a more depressive mood and don't always have the motivation to make healthy choices.
As hard as it feels to start, exercise helps. A healthy diet with plenty of vegetables also helps, in fact much of your body's serotonin is actually produced in your gut.
If you're motivation is low and you're having trouble getting to the gym or even grocery store, try some citrus essential oil. Citrus often lifts our mood and can at least temporarily help get us off the couch. Some citrus can also benefit immunity and digestion so it's a great all-around oil to have on your shelf for this completely blah time of year.
Here's some additional reading about a healthy late-winter lifestyle, and info on Seasonal Affective Disorder. Psychology Today site also has a search function for therapists in your area if you'd like additional support.