drop off

to start to sleep

If the amount, number, or quality of something drops off, it becomes less:
The demand for mobile phones shows no signs of dropping off.

アメリカ英語辞典
to begin to sleep:
I must have dropped off during the show, because I don’t remember how it ended.

to become lower in level, value, price, etc.:
Business dropped off a little during the summer.
The company's growth dropped off steadily to 0.3% last year.
The telecoms' downturn meant that potential sales and profits dropped away.

a reduction in the amount or level of something:
Analysts say the employment drop-off is directly linked to a slowdown in housing construction
a drop-off in sth There was a sharp drop-off in industrial production in December.
a slight/steep/significant drop-off
expect/notice/see a drop-off

an act of taking people, goods, mail, etc. to a place, or the place where you take them:
Passenger drop-off is allowed only in the designated area.
recycling/waste drop-off Take anti-freeze, paint thinners, or other chemicals to a hazardous waste drop-off.
drop-off area/centre/point

drop offの例
There is a special protected drop-off lane on the eastbound side.
- Cambridge English Corpus

The result of this last process is a gradual drop-off in reaction time and error rate.
- Cambridge English Corpus

- Cambridge Dictionary