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1. The easiest (and you'd be surprised how much it can make right, especially battery issues) is the simulated battery pull. You do this by simply holding down the power button. Ignore the shutdown menu that comes up and keep holding the power button until the Nexus 5 spontaneously reboots.
2. The next easiest is to boot into the stock recovery and wipe the cache partition.
2. If neither of the above work, do a factory reset. This will wipe everything except your internal storage and you'll have to redo everything to set up your Nexus 5 again. However, I tend to do this routinely after a major OS OTA upgrade.
Good luck. Hope this helps.