You better take that rubber band off your wrist before you go to sleep otherwise it could stop your blood from flowing.
You shouldn't sleep with that elastic band on your wrist. It could stop your blood circulation.
ーYou better take that rubber band off your wrist before you go to sleep otherwise it could stop your blood from flowing.
「寝る前に手首のその輪ゴムを取らないと血が止まっちゃうよ。」
take that rubber band off your wrist で「手首のその輪ゴムを取る・外す」
it could stop your blood from flowing は「血が流れるのを止めるかもしれない」＝「血（の流れ）が止まるかもしれない」と言えます。
ーYou shouldn't sleep with that elastic band on your wrist. It could stop your blood circulation.
「手首にその輪ゴムをつけて寝ちゃダメだよ。血の流れを止めちゃうよ。」
sleep with that elastic band on your wrist で「手首にその輪ゴムをつけて寝る」
blood circulation で「血行・血のまわり」
ご参考まで！
カナダ 
