This is one of my gripes about working in an office. More often than not I cannot work continuously from 9-5, and end up wasting time just trying to get myself back into working mode - which usually descends into browsing HN/Reddit/etc or just working half as effectively as I usually do.
When I work remotely and have freedom over my time, I can take a few hours off to go for a walk, sleep, go for a long lunch, meet friends, or even play games. Afterwards I feel recharged and can be 110% productive, and usually end up working more hours than I would in an office.
I did this for a lot of last year, as I needed to overlap with teams in India and PST, so I usually worked 10am - 10pm taking a few hours off in the afternoon. I was able to get so much more done because of that extended break.