If the room is too hot or dry you may not be able to last as long safety. A sleepsack that is too thick or hard (ones made for suspension etc) or one with a backbone might be too uncomfortable for long term wear but this depends on the sub as well.

Something I suggest is a little cushion between the knees. This person makes one specifically for sleepsacks but you could improvise easily

https://www.etsy.com/listing/1308345662/leg-knee-pad?click_key=d2d2d08baa499580ec8e2f8f3e7bdfe7799f07fc%3A1308345662&click_sum=04caaa16&ref=shop_home_recs_22&frs=1&sca=1