成唯識論演祕 3.924

3.924

解云：即於前位身心惛昧，故至後念心等不行。此說所因非無心位，然此立因應云身心極惛昧故，不爾不定。有心睡眠亦惛昧故。

The explanation states: it is precisely in the previous state that body and mind were stuporous, and therefore in the subsequent thought-moment mind and its associates do not operate. This speaks of the cause and does not refer to the mindless state. However, in formulating this reason one ought to say 'because body and mind are extremely stuporous'; otherwise it would be inconclusive, since sleep with mind is also stuporous.