After installing few windows updates on the Windows 7, my system started behaving weirdly that it wakes up automatically within few minutes after I put the system in sleep mode.
After some research and troubleshooting following solved my problem
Device Manager à Network adaptersà wireless adapter (select speific wireless) and uncheck the “Allow this device to wake this computer” option
Resources
http://windows.microsoft.com/en-US/windows7/Sleep-and-hibernation-frequently-asked-questions