Just because your smartphone or tablet (or smartwatch for that matter) looks like it's doing nothing, that doesn't mean that there's nothing going on. If it's connected to a network, there are communications going on that constantly wake it up from sleep mode, and eat at your battery life.
These communications come in the form of Router Advertisement (RA) messages, and according to a Internet Engineering Task Force (IETF) memo RCF 7772 by Cisco engineer called Andrew Yourtchenko and Google researcher Lorenzo Colitti, if these are sent too frequently, they can put a huge dent in battery life.
"Current-generation devices might consume on the order of 5mA when the main processor is asleep," the pair write. "Upon receiving a packet, they might consume on the order of 200mA for 250ms, as the packet causes the main processor to wake up, process the RA, attend to other pending tasks, and then go back to sleep. Thus, on such devices, the cost of receiving one RA will be approximately 0.014mAh."
Doesn't seem like much, and it isn't for notebooks and desktop PCs. But when you consider that the Apple Watch only has a 205mAh battery to begin with, that's only about 14,500 RA messages worth of power. Sounds like a lot, but it's not when you consider that networks can be configured to send RAs every few seconds.
Yourtchenko and Colitti claim that this sort of power consumption is too high, and say that they should consume no more than 2 per cent of a device's power during sleep mode. Their target is that "the average power budget for receiving RAs must be no more than 0.1mA, or approximately 7 RAs per hour."
The memo also contains recommendations on how network admins can configure routers to better serve the devices connected to them.