A cheap kitchen timer or anything that can remind you to take a short break should work. If you can set an alarm for every hour that will remind you that you have the option of taking a break. Also, making an effort to take a proper lunch break every day when possible is something that has helped me.
2. I can do all day long: calls, computer, meetings. Finish one action, start the next. And so on till the day ends. Without any rest. The only problem here is I feel tired and have headaches by the end of the day. I don't feel when I need to have rest. I used to smoke and used that as a rest period. Quit a few years ago so have no 'natural' rest periods now. Any recommendation on how to setup a rest period and should I have rest during work hours?
I think that thinking about the Next Action as the very next, visible physical action, is helpful. In that perspective, "think about" is not a suggested phrasing as it is not something visible. Can you visualize yourself doing the "thinking"? If so, what will you see yourself doing? And: What is the intended result of your "thinking about X". I find that, for me, in many cases, "think about" really means "talk to Y about X", "mindmap X", "look up this and that about X" etc.
3. My actions mostly fall under @Calls and @Computer. I also have a lot of "think about" next actions. Think about St Valentine's day present for my wife, think about birthday present for my wife, think about what to do next for project ABC etc. Which context should it fall under?