Brain fog affects over a quarter of us — yet it isn't actually a medical diagnosis. So what's really happening inside your brain when thinking feels like wading through treacle?

In this episode, Jen, Chris, and Matt unpack the real biological mechanisms behind brain fog — from neuroinflammation and a compromised blood-brain barrier to mitochondrial dysfunction and hormonal shifts. The team explains why your brain's immune cells can get stuck in "emergency mode," how chronic stress physically shrinks the part of your brain responsible for memory, and why up to 95% of serotonin is produced in your gut, not your brain. Most importantly, they explain why brain fog is typically reversible once you address what's causing it.

In this episode:

1. 00:00 — Introduction and what brain fog actually is
2. 01:38 — Your brain's energy demands and neuroinflammation
3. 03:24 — The blood-brain barrier and why it leaks
4. 05:35 — Mitochondria and the vicious cycle
5. 07:04 — Sleep and the brain's rubbish collection
6. 08:15 — How chronic stress shrinks your hippocampus
7. 09:34 — Hormones, menopause, and thyroid
8. 11:30 — Dehydration, gut health, and nutritional deficiencies
9. 14:30 — What you can actually do about brain fog
10. 17:33 — Why everything is connected
11. 19:01 — So that's why we get brain fog

Your Brain's Immune Cells Can Get Stuck in Emergency Mode

(01:38)

One of the major drivers of brain fog is neuroinflammation — when the brain's immune cells, called microglia, become activated and stay activated long after the initial threat has passed. These cells keep releasing inflammatory molecules called cytokines, which disrupt normal brain signalling.

"Studies using brain imaging found that patients with post-COVID brain fog show widespread microglia activation — their brain's immune cells are basically still in this emergency mode, months after the infection." — Chris

The degree of inflammation directly correlates with symptom severity — the more inflamed the brain tissue, the worse the cognitive symptoms. Those cytokines impair memory, attention, and mental clarity.

This connects to the blood-brain barrier — the membrane that controls what enters the brain from the bloodstream. A 2024 study found that in long COVID patients with brain fog, this barrier becomes compromised, allowing inflammatory molecules and cells that should never reach the brain tissue to get in.

"The security gate isn't just ajar — it's actually letting through some suspicious characters that really shouldn't be getting in." — Matt

The barrier disruption was most notable in the temporal lobes and frontal cortex — regions crucial for memory and attention.

Sleep Deprivation Cancels Your Brain's Rubbish Collection

(07:04)

Sleep sits at the top of the brain fog trigger list. Research shows that after just one night without sleep, reaction time slows by 50%, working memory drops by 40%, and the ability to form new memories decreases by nearly 40%.

During sleep, the brain goes into maintenance mode — flushing out waste products, including proteins that can contribute to conditions like Alzheimer's. Brain function doesn't typically return to normal until roughly 72 hours after sleep deprivation.

"Pulling an all-nighter essentially cancels the brain's rubbish collection." — Matt

Chronic stress compounds the problem. Cortisol — the body's main stress hormone — can physically shrink the hippocampus when it stays elevated for too long. Research has shown that people with chronically high cortisol levels often have a smaller hippocampus and perform worse on memory tasks. And because the hippocampus also helps regulate stress, damage to it means the brain can't keep cortisol in check — creating a vicious cycle.

Hormones, Gut Health, and the Triggers Most People Overlook

(09:34)

Oestrogen acts as a neuroprotective hormone in the brain — supporting memory, enhancing blood flow, and protecting cells from damage. During perimenopause and menopause, when oestrogen levels drop, many women experience brain fog for the first time. Studies have documented measurable cognitive changes during these transitions.

"Women aren't just being emotional or stressed. Their brains are actually responding to dramatic shifts in a hormone that's been protecting and supporting their cognitive function for decades." — Jen

Thyroid hormones are equally important. Nearly 80% of people with hypothyroidism report frequent brain fog symptoms — the direct result of brain cells operating at a slowed metabolic rate.

The gut-brain connection adds another layer. The brain and gut communicate bidirectionally through the gut-brain axis, and up to 95% of serotonin — crucial for mood and cognition — is produced in the digestive system. Studies have shown that patients with irritable bowel syndrome often experience brain fog, and treatments that help rebalance the gut microbiome can improve cognitive symptoms.

Brain Fog Is Reversible — Here's What Actually Helps

(14:30)

The encouraging news is that for most people, brain fog is not permanent cognitive decline — it's a reversible state that improves when you address the underlying causes.

1. Sleep — Most adults need seven to nine hours. The brain needs that time for maintenance, waste clearance, and memory consolidation.
2. Hydration — The brain is over 70% water. Even mild dehydration impairs cognitive function, but effects reverse quickly once you rehydrate.
3. Nutrition — Omega-3 fatty acids, B vitamins, vitamin D, and natural antioxidants all support cognitive function. Limiting processed foods that trigger inflammation is also worth considering.
4. Stress management — Chronic stress physically damages the hippocampus. Finding what works for you — mindfulness, breathing exercises, walking outside — helps break the cortisol cycle.
5. Exercise — Physical activity increases cerebral blood flow, supports mitochondrial function, reduces inflammation, and regulates stress hormones.

"Brain fog's telling you that something needs attention. But your brain is incredibly resilient. If you address the underlying cause, give it the support, your cognitive function typically does improve." — Jen

For brain fog that coincides with perimenopause, menopause, or thyroid issues, speaking with a doctor is recommended — there are treatments that can help.

About So That's Why

So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.