Grief raises cortisol and keeps the stress system switched on. That alone disrupts sleep, appetite, digestion, and immune function.
Memory and concentration suffer because working memory is being used by rumination and emotional processing. This is real, measurable impairment — not you failing.
Attachment systems in the brain keep predicting the person's presence. That's why you reach for the phone, hear their car, or expect them in the doorway.
Broken heart syndrome — takotsubo cardiomyopathy — is a documented physical heart response to acute grief. The phrase was never only metaphor.
Practical translation: expect to be slower, hungrier or less hungry, more forgetful, more susceptible to colds, and worse at decisions. Plan around it rather than judging it.