What is grief brain?
Grief brain is the temporary fog, forgetfulness, and poor concentration that commonly follow bereavement: losing words, rereading the same page, missing appointments. It is a normal stress response, not cognitive decline; the grieving brain is working hard on the loss, leaving less capacity for everything else. It typically eases as acute grief softens.
People arrive at this search scared: I put the keys in the fridge, I forgot my own meeting, I cannot follow a paragraph, is something wrong with my brain? Almost always the answer is no. Something is wrong in your life, and your brain is responding accordingly.
Why loss fogs the mind
Three drains run at once. Grief is cognitively expensive: beneath the surface, the mind is doing the enormous work of updating a world model in which the person no longer exists, and that work does not show up on any to-do list. Stress physiology lightens sleep and floods the system, and both poor sleep and stress hormones degrade attention and memory on their own (if 3am is part of your story, see grief and sleep). And intrusive memories interrupt: mid-email, the hospital corridor arrives uninvited, and the email loses.
What helps while it passes
Lower the bar and add rails. Write everything down, not as defeat but as a cast on a broken arm: lists, alarms, notes on the door. Shrink tasks to twenty-minute pieces with breaks; a foggy brain finishes small things and stalls on big ones. Tell one person at work the truth ("my concentration is off since the death; deadlines may need slack for a while"); most people respond better to honesty than to mysterious dropped balls. And treat sleep as the active ingredient it is.
When to get it checked
See a doctor if the fog is getting worse rather than better after several months, if it is joined by daily hopelessness or an inability to feel anything at all, or if you simply want reassurance; ruling things out is a legitimate reason for an appointment. Persistent fog can travel with depression and with prolonged grief disorder, which affects roughly one in ten bereaved adults and responds well to grief-specific therapy. The fog lifting is the norm. Help exists for when it does not.
See also: Lundorff et al. (2017), Journal of Affective Disorders, meta-analysis estimating prolonged grief in roughly 10% of bereaved adults; Stroebe & Schut (1999) on the cognitive work of oscillation.