Deep breathing can have an immediate and profound effect on one's physiology. It can relax the fight-or-flight sympathetic nervous pathway and activate the parasympathetic path way. Harvard Medical School Dr. Herbert Benson brought attention to the positive effects of breathing in his 1975 book The Relaxation Response.  

Buddhist monk Thich Nhat Hanh has a perspective on mindful breathing that I find very accessible.  Wherever you are, you can turn your attention to your breath.  No fancy technique is required to get started. In his books, he suggests using everyday prompts to remind us to breathe. Here, I am offering the image of the 3 plants as a quiet, wordless reminder.

Further on-line reading:
David DiSalvo. Breathing And Your Brain: Five Reasons To Grab The Controls. Forbes.com. 2013. http://www.forbes.com/sites/daviddisalvo/2013/05/14/breathing-and-your-brain-five-reasons-to-grab-the-controls/

Permission to Rest is a reference to not feeling guilty about sleeping and taking short breaks.  

Here are some reasons why we can feel good about sleeping an optimal amount, about 8±1 hours a day, which can include naps.

• Better physical health - including a stronger immune system, and easier weight maintenance.
• Improved cognitive functioning - including better creativity, memory, and productivity. 
• Reduced anxiety.
• Better problem solving.  At night we process the information gathered through the day, solving unresolved issues. Hence the phrase "Sleep on it".

We also benefit by taking short breaks from focused tasks. Our brains have a "focused mode" and a "diffuse mode". Diffuse mode might feel like daydreaming but it is actually using a different part of our brain to tackle problems.  We can all relate to times when ideas come to us in the shower or when driving.  This article by Courtney Seiter shares some of the research about taking breaks and provides an overview of techniques.
Link here: https://open.buffer.com/science-taking-breaks-at-work/

For me, the Serenity Prayer is about accepting one's emotions and believing in the possibility of behavioural change, even when change is difficult. 

The inspiration for the piece probably came from my Taoist upbringing. However there is a form of cognitive behaviour therapy called Acceptance and Commitment therapy which uses acceptance and mindfulness strategies to gain psychological flexibilty.

I've seen this image over a hundred times, and it still makes me chuckle - everytime.  I use it here as a demonstration of how a pun asks us to shift perspectives and exercise some mental dexterity.

The emotion of humour is called mirth and when we experience mirth our brain releases dompamine, feeding our pleasure centers. Mirth also releases endorphines that produce a relaxation response. 

Further readings about humour at this PBS page. 
http://thisemotionallife.org/topic/humor/humor