Life can be stressful, and sometimes, the after-effects of a stressful life event make themselves known even after the episode has passed. It’s almost as if our nervous systems ratchet up to brace ourselves for a big day — and then forget to uncoil.
Omega-3 fatty acids have been well-studied for their role in supporting mental health, particularly in regards to promoting a positive mood.* Now, a new study indicates the omega-3 DHA could help folks’ nervous systems relax after a stressful event. 
Japanese researchers studied 83 people suffering from episodic stress. Participants were randomly assigned to take either a DHA supplement (providing 1,470 mg of DHA per day plus 147 mg of EPA) or a placebo pill for 12 weeks. At the end of the study, those taking the DHA enjoyed a calmer heart rate than those taking the placebo, even when asked to visualize a stressful situation.*
How does DHA work? The researchers think it has a calming effect on the autonomic nervous system (which governs basic bodily functions not under conscious control, such as breathing and heart rate) rather than the sympathetic nervous system (which initiates the stress-provoked flight or fight response).*
Specifically, they suspect that DHA increases the activity of the vagus nerve.* The longest cranial nerve in the body, the vagus nerve helps control heart rate through a series of electrical impulses.* The more active the vagus nerve becomes, the more it calms heart rate.* This finding jives with previous research, which has shown that omega-3s increase vagal activity. 
 Matsumura K, et al. J Affect Disord. 2016 May 30. Doi: 10.1016/j.jad.2016.05.054 [Epub ahead of print]
 Christensen JH. Front Physiol. 2011;2:84.