MacBook Pro won't wake up sometimes

For me upgrading to macOS Sierra resolved the issue. I tried resetting NVRAM and SMC but that didn't help.

Update 8/2/2018: turns out this issue is still happening to me now, just much less frequently, about once every few months.

As a starting point it's worth actually resetting both your NVRAM and SMC.

Reset NVRAM

Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). To reset this on your MacBook Pro, follow these steps:

1. Shut down your machine. Yes, a full shut down, not just logging out.
2. Using your built-in keyboard, press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
3. Hold those keys down until your MBP reboots again and you here the startup chime.
4. Let go of the keys and let your MBP reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Reset SMC

Your computer's System Management Controller (SMC) can also be reset. The instructions for resetting the SMC on your particular MBP are as follows:

1. Shut down your MBP
2. Keep the MagSafe adapter (power cable) plugged in.
3. Using your built-in keyboard, press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
4. Let go
5. Turn your computer back on with the power button.

Let me know what change in behaviour you notice (if any) upon rebooting.

Also, it may be worth you reading Use your Mac notebook computer in closed-display mode with an external display.