入睡用英语可以说作"to fall asleep"。这个说法的来源可能源于人们在睡觉时身体的放松和重心的下沉，使人感觉像是"falling"入睡。例句如下：

I always have trouble falling asleep at night.（我晚上总是很难入睡。）

I fell asleep as soon as my head hit the pillow.（我一头靠在枕头上就睡着了。）

It's hard for me to fall asleep if there's too much noise.（如果太吵，我很难入睡。）

此外，入睡也可以用其他说法表达，例如"to drift off to sleep"（渐渐进入梦乡）、"to nod off"（打盹）或"to doze off"（打瞌睡）。例句如下：

I always drift off to sleep when I'm on the train.（我坐火车时总是渐渐进入梦乡。）

I nodded off during the movie last night.（昨晚我看电影时打了个盹。）

She dozed off in her chair and didn't wake up until the next morning.（她坐在椅子上打了个瞌睡，直到第二天早上才醒来。）