To fall asleep very quickly or to lose consciousness suddenly

अचानक सो जाना या बेहोश हो जाना

English Usage: After a long day at work, he was out like a light as soon as his head hit the pillow.

Hindi Usage: काम के लंबे दिन के बाद, उसका सिर तकिए पर रखते ही वह तुरंत सो गया।