Well, this question might sound stupid, but I don't know what else could have happened. I had some issues with internet connection, so I phoned to my ISP, who said that they had changed something which was causing the problem and now it's fixed. But until this, when my phone was in standby and wifi turned on, there was a huge difference in battery life between Wifi off and on. The phone drained 2% an hour when wifi was on and when off it was ~0.5%. Now, however, after changing some connection settings by the ISP, the connection is slower, but on the other hand my phone stanby was dramatically improved using wifi. It's something like 0.75% per hour. Is it possible that the change is the cause or is it due to something else which I didn't notice?