Default [Q] Random Vibrations and Screen Wakes without Notifications

I've just received my Nexus 5 earlier today and, coming from my old Samsung Galaxy Nexus, I'm really loving it so far. However, I've noticed that, at random, my Nexus 5 will vibrate and the screen will wake, sometimes once, sometimes twice in a row. There are no notifications following the vibration and screen wake, and no real indication at all as to why this may be happening.

At first, it started with Hangouts vibrating and waking the screen four to five times after each new text message I would receive, until I finally unlocked the phone and opened the Hangouts app. I would get an SMS text, the phone would vibrate and the screen would wake. Then, just after the screen would go back off on its own, the device would vibrate and the screen would wake again. This was a recurring issue today, so I had assumed it was just a bug with the Hangouts app (which did not at all surprise me).

However, for the past several hours, I have not received any text messages or Hangouts messages at all, yet at random times my phone will vibrate and the screen will wake with no notifications or other indications as to why. Sometimes it will happen once, followed by a second occurrence of this behavior about a minute later. Other times, it will happen twice in succession. I was wondering if maybe it was Google Now, but I had disabled vibration on notifications for Google Now. I also considered Gmail, but notifications are turned off entirely for my Gmail app.

I'm at a loss here, and I'd really appreciate any insight into this issue. For those wondering if it might have something to do with the battery overheating and the system trying to notify me about it, the battery has been keeping below or around 80 degrees Fahrenheit for the vast majority of the day, so I doubt this is the case, though I'm open to any and all possibilities.

Other helpful information:
My Nexus 5 is running stock Android KitKat 4.4.2, purchased new directly from the Google Play store online.