Every assignment you work will have the main objective of maintaining the security and safety of a site and its contents (people or otherwise). With shifts lasting up to 12 hours each, you will of course need to take breaks.
It is very important that you follow the specific rules of the company you are working through - make sure you agree with them the breaks you can take - agreement should include:
When to take the break(s)
how long the break(s) should be
where you are allowed to take the break(s)
if you are allowed off site
Following these simple rules will ensure the effective security and safety of both you and the site.