The Airtel unlimited data plan for night browsing comes in three categories just like the hourly based plan:
1. 1hour plan for N100 – dial *481*1# to activate.
2. 3hours plan for N300 – dial *481*2# to activate.
3. 6hours plan for N500 – dial *481*3# to activate.

Time frame for the unlimited browsing on the Airtel network is from 12AM to 6AM every day.
The plan works for all devices starting from Android Smartphones, iPhone/iPad, Laptops, BB10, Windows phone, Symbian/Java phones; to any you can think of.


Source:: http://news.dpfaces.com/airtel-introduces-unlimited-night-data-plan/