When winter comes, there are fewer daylight hours. As a result, the amount of serotonin in the body decreases, which might cause us to feel depressed.
「冬になると」は直訳的に「when it becomes winter」になり、それは通じますが、英語で大体When winter comesやwhen winter startsをよく使います。
there are fewer daylight hours
日照時間が減る
As a result, the amount of serotonin in the body decreases,
その影響で体内のセロトニンの量が減って
which might cause us to feel depressed.
鬱っぽくなるのかもね
ご参考になれば幸いです。