YOU SAID:

I don't have the mental capacity to sleep

INTO JAPANESE

眠る精神力がない

BACK INTO ENGLISH

I don't have the mental capacity to sleep.

INTO JAPANESE

私には眠る精神的能力がない。

BACK INTO ENGLISH

I have no mental ability to sleep.

INTO JAPANESE

私は眠る精神的能力がない。

BACK INTO ENGLISH

I have no mental ability to sleep.