Delhi used to have strict time limits for restaurants and bars. Usually you would have been hard pressed to find anything decent after 11:00pm at night. It was common for people to go to NCR towns for some late night partying and drinking. Things have improved subsequently and now there are several alternatives for dining in late hours.

One of the first 24 hour restaurant in Delhi was Cumsum that opened near Nizamuddin Railway station in South Delhi. The food is just about passable but it is the flexibility of eating say even at 4:00am in the morning thats its main calling card. Then there has always been Pandara Road. A small market block, it has several good restaurants that stay open upto 2:00 am in the night. A lot of Five Stars hotels these days offer midnight buffets. Though slightly on the expensive side, you can be assured of the quality of food.

There are many bars and restaurants in Saket that are open till late too like the Spice Market, Urban Cafe, V Spot Cafe Bar and Flo.  

Barista is the brand of Starbucks style Cafes in Delhi. Some of it's branches stay open through the night, like the one at PVR Complex - Saket. In general, the restaurant timings have shifted towards mid night in Delhi. You can place your last orders by about 11:30pm in most places. If you have the stomach for some tasty street food, head towards Roadside 'Dhabas'  which dot all the national highways leading out of Delhi. 

Another option is Route04 and boombox cafe in khan market. They are open till 12:30am .

McDonalds in rajouri garden and saket are open till 1:00am.