In that situation, will/going to are not so different. You could say either and, in real-life situatins, no-one would notice either way. The grammatical way to look at it is if we are planning in advance. If it is the morning and you feel tired form a poor night's sleep, you might say "I feel tired so I am not going to work overtime" If you make the decision at the time (a few minutes before overtime) and it is more spontaneous you should say "I feel tired so i won't work overtime"

You CAN use the present progressive/present continuous to talk about the future as well as the present. However, it's more clear that you are talking about a future action if you add an adverb of time (tonight, tomorrow, next week, this summer, etc.). Examples: What are you doing this summer? I'm going to the movies this weekend. Next week, I'm going to work overtime. I'm going to bed early tonight.

If the action (going to bed etc.,) is in the future then you need to use the future tense. For example: She is tired, (therefore), she will go to bed earlier (than usual)- or - she will be going to her bed earlier than she usually does or usually would do. I'm tired (so) I'm not doing any overtime today - or I won't be working overtime today - or there's no way I'll be doing any overtime today or the boss can take his overtime and stick it where the sun doesn't shine. If you say "I am" doing something then it means that you're doing it now, not later. In other words, "I am", "she is" and similar constructs do not imply intent: instead, they describe a current action.

All correct and mean the same: She is tired. She will go to bed earlier. She is tired. She is going to go to bed earlier. She is tired. She will be going to bed earlier. She is tired. She is going to be going to bed earlier. (sounds really odd, but it's gramatically correct. I can imagine little kids saying something like this when playing with each other). I'm tired. I won't work overtime today. I'm tired. I am not going to work overtime today. I'm tired. I won't be working overtime today. I'm tired. I am not going to be working overtime today.

Ooops! Meant to add this as an answer, not a comment :)