It's comfortable for me sleeping flat on my back with my knees bent up in the air.
A good position for me to sleep in is flat on my back with my legs bent upwards.
ーIt's comfortable for me sleeping flat on my back with my knees bent up in the air.
「仰向けで膝を曲げて寝るのが楽だ。」
sleep flat on my back で「仰向けで（平らに）寝る」と言えます。
with my knees bent up in the air で「膝を曲げて」
ーA good position for me to sleep in is flat on my back with my legs bent upwards.
「私にとって寝るのに良いポジションは、仰向けで足を上向きに曲げて寝ることです。」
with my legs bent upwards でも「足を上向きに曲げて」＝「膝を曲げて」と言えます。
ご参考まで！
カナダ 
