Battery life is one of the weaknesses of today's smartphones. 2016 devices come dripping with radios, high resolution screens, multi-core processors with gigabytes of RAM and running complicated, sophisticated applications, usually designed with working as fast as they can rather than sipping at battery. Google has recently declared war on battery life and this is a very good thing, because even with Android 6.0 Marshmallow's power saving technologies, stock Android on a Nexus device is typically less efficient than a manufacturer-modified software build. There are many reasons for this, but the long and short of it, is that manufacturers deliberately tailor the software on a device to give the best user experience and this includes battery life. Google's stock Android has a different set of priorities.
There are several manufacturers associated with providing solid battery life and Sony are definitely one of them. Sony launched the STAMINA battery preservation mode some years ago and had been steadily improving it over the years. STAMINA mode is the obvious front end to a number of changes under the skin of a Sony device, including batch network processing controlled by the operating system, and the ability to disable radios in order to prevent applications from connecting and doing their thing whilst the handset is in standby. STAMINA includes an application white list, that is, applications that will always be allowed a network connection and for many customers, thanks to this function, STAMINA could be left running all of the time.
However, Google's Android 6.0 Marshmallow broke how Sony used STAMINA mode because under the terms of using Marshmallow, certain features cannot be adjusted - including how Doze and App Standby work. The rub here is that Google's power management applications are not as good as Sony's rather brutal, but effective, STAMINA mode. For example, Doze does not shut things down on the device until it has been idle and unmoved for a period of time. In contrast, STAMINA takes effect almost immediately.
When Sony started updating its fleet of devices to Android Marshmallow, it lost STAMINA and customers complained. However, it did return in the latest Xperia Z5 update. Although, unfortunately it does not seem to be back in its original form: Sony appear to have rebranded the stock Android Power Saving mode, which disables certain features (such as animations, vibration feedback, location services and background data) whilst limiting screen brightness and processor performance. It's a blanket ban on background data using the operating system but there's no white list, so it means that messaging applications won't work. And it is not STAMINA mode by a long shot.
Presumably, Sony felt immense pressure from customers who wanted STAMINA back, but also did not want to break Google's terms and conditions for using Android 6.0 Marshmallow. Quite where Sony will go now with their device software remains to be seen, but as it stands, the new STAMINA mode is simply not as effective or as usable as the old way.