If you're like me, the fall equinox is the beginning of a dark, sad time. We're not alone. Seasonal Affective Disorder (SAD) affects many people, depending on latitude (more people the farther north you go, until you hit Iceland). Once seen as a disorder in its own right, now SAD is considered a seasonal form of depression.
As you might think, a mood disorder brought on by short day length often responds to light therapy. When phototherapy was begun, it often involved large banks of light which were expensive and awkward. Getting more light has become much easier for me since I found the Syrcadian blue LED light. This is a compact bar of LEDs tuned to the blue part of the spectrum that is thought to be involved. It is powered by a USB plug (I use a portable battery or my laptop, depending on what I'm using) and it sits nicely on top of a laptop computer or an iPad. It would probably also nest nicely on top of a book.
I typically start using it on the equinox, turning it on when I get up and using it (with a break for breakfast) until 9:30 am. I found that it feels good, so good that if I don't set a time to turn it off it's easy to get a mild insomnia later on. I found it so handy that I offered ones to my adult children - they both have found it useful, too.
Not everyone with SAD finds light therapy enough help. The linked Wikipedia article discusses some other ways to treat SAD, and they can be used alone or as adjuncts to light therapy.
Here's wishing you clear skies and brighter days!
Update: as so many folks have mentioned other modalities in the comments, I'll add some up here. While light therapy is all that some folks need, others can benefit from exercise, vitamin D or a dawn lamp that simulates the natural sunrise, either alone or in combination with the light therapy or each other. There are useful links for all of these in the Wikipedia article above. Some folks need anti-depressants but those will need a physician's prescription.
Some folks have the reverse problem: fall and winter are fine, but light and heat are their burdens. Do feel free to share any strategies that have helped with that, too!