Insta-wakes are when the Mac awakes within seconds (sometimes less) from being told to go to sleep.
There might be several reasons to why your Mac instantly awakes after being instructed to sleep. Select the "Insta-wake" event in the calendar, and observe the information presented in the information panel on the right.
Should list the reason as to why the Mac woke.
If there is any software that may be responsible for the wake, it will be listed here.
Lists connected devices, try disconnecting these devices to see if it makes any difference for later sleeps.
Typically these shouldn't cause it wake so quickly. However they're reported so they can be compared with subsequent sleeps.
Really needs to be chosen based upon what is causing it to wake. Sometimes the Mac will tell you via "Woken By" field at the top of the information, while others will have to be determined through trial and error.