Number formatting: use of digit separators in manuscripts I am reading through a manuscript that I am co-authoring with a colleague and I noticed that it used digit separators/marks for all the numbers (i.e. 2,500 instead of 2500). Maybe it's just me but I think this style started after my colleague started doing a post-doc in the U.S. 

I have checked the author guidelines of a couple of journals that are likely submission targets in our field and they don't seem to include anything about decimal/digit separators. I also checked my previously published articles and noticed that numeric values were not edited (by press editors that is) to include digit separators. 

Thus my question, is there a general rule-of-thumb regarding number formatting, especially considering digit separators?
Digit separators differ between countries, particularly since the decimal seprator is comma in many countries but a period in English/American. Hence in English you may use commas as digit separators while in other countries periods are used. Space is therefore the only separator that is not confusing. A general rule of thumb is to not use separators for single-thousands but start to use them for tens of thousands and up

1000
10 000
100 000
1 000 000

When using spaces in writing, it is good to remember to use non-breaking spaces so that numbers are not broken over lines.