I'm too tired to work overtime safely.
 
I'm sorry I'm not available to work this weekend.  I have plans with my family.
I'm feeling overwhelmed.  Can you give the admission to someone else or help me in some other way?

1. Be in tuned to how you feel.  Are you stressed? Tired?  In pain?  All of these may be common and present and what constitutes too much stress, fatigue, or pain varies among individuals and over the course of a shift and lifetime.  Watch the "Interruption Awareness" ¹  youtube to see some valuable teaching moments of what stress looks like and the "Overload" activity is a fun way to develop your
awareness about how you are feeling in a fun way!
 
2. Honor what you are feeling.  You are the only one who can really gauge it.  Just as we are taught not to tell patients, Don't worry, or Don’t be anxious, it is much better to validate.  A simple internal acknowledgment such as; Hmmmm, I'm feeling sad, (or frustrated, or worried etc) is an important step in understanding and managing your feelings.
 
3. Use ownership language.  The above examples show ownership and I-statements ²  can be helpful.  Keep in mind that you need not apologize for your feelings or explain them although this is a judgment call.   Apologizing for not being available is different from
apologizing for feeling fatigued or stressed.  Offering a little more information such as the number of overtime hours you've already worked or what plans you have with your family may be appropriate and honoring of your role as a team member and part of the larger organization.
 
4. Let the silence be.  Once you set a limit yet in this space of silence there is opportunity for others to help.  It is also a place where compromise and co-creative problem, resist the urge to retract it or fill in the silence. If setting limits is new for you, you might surprise others who are used to your willingness to fix, help, and problem-solve.
 
5. Try trusting the village.  I know that there are toxic teams and workplaces out there and would never insist on this tip.  However, there may be others who are willing to help if given a few moments to think about it.  By setting your limit and letting it hang 'out there' you might be surprised by who steps up to the plate.  And if no one does, you'll have more information about the culture in your team and the toll it might be taking on you.
 
6. Consider compromise.  Compromise can be a healthy process provided it isn't a chronic pattern and doesn't interfere with solving underlying problems or lead to working unsafely.   Offering to work additional hours from time to time may be part of a work expectation and let's face it, sometimes staffing crunches or excessively busy times will happen. And in reality sometimes you'll be able to help and sometimes you won't. 
 
7. Accept some anxiety during and after the process.  There is often a perceived emotional risk in saying No etc. and during the silence you may feel your heart racing.  Just note it and take a deep slow breath.  You may also feel insecurities arise after someone has honored your limit or you've reached a compromise with others.  This may be a rich opportunity for you to learn more about what motivates you to help others.  Do what ever works for you to manage stress.  Take a deep breath, go for a jog, or journal about your experience are some ideas.  It's new emotional territory and very normal to be a little anxious.  If you find yourself excessively anxious, get some help from a counselor talk with a trusted friend.  It is good to get to know and take care of yourself!