YOU SAID:

I always get sleepy when it gets dark.

INTO JAPANESE

暗くなるとき、私はいつも眠くなります。

BACK INTO ENGLISH

When it gets dark, always makes me sleepy.

INTO JAPANESE

暗くなるとき常に眠くなります。

BACK INTO ENGLISH

Always sleepy when it becomes dark.

INTO JAPANESE

暗くなるときに常に眠い。

BACK INTO ENGLISH

Always sleepy when it becomes dark.