Set aside the thought ______ you have to fall asleep为什么横线填that不填before或when？

Set aside the thought ______ you have to fall asleep为什么横线填that不填before或when？

这里是feel 感觉，不是fall。
feel sleepy 感到困倦
fall asleep 入睡。
这俩注意下动词的区分。追问Set aside the thought ______ you have to fall asleep为什么横线填that不填before或when？追答that引导的同位语从句 you have to fall asleep是thought的内容。