To prepare for sleep in English, you can say "I'm getting ready for bed" or "I'm going to bed now." These phrases are commonly used to indicate that you are preparing to go to sleep.

The origin of these phrases likely comes from the practice of actually getting ready for bed, such as brushing your teeth and changing into pajamas, before climbing into bed to sleep.

Here are some examples of these phrases in use:

"I'm feeling really tired, so I think I'm going to bed now." (我感到非常疲惫，所以我想现在就去睡觉。)

"I'm going to hit the hay. See you in the morning!" (我要去睡觉了。早上见！)

"I'm getting ready for bed. Goodnight!" (我正在准备睡觉。晚安！)

Note: "Hit the hay" is another common way to say that you are going to bed. It is a colloquial expression that likely originated from the practice of sleeping on a hay bed.