When the Mind Won’t Turn Off: How Overthinking May Be Tied to Alzheimer’s

What is the Default Mode Network?

Imagine your brain as a city. Some parts light up when you’re doing something active—like problem-solving or listening to someone talk. But when you’re resting, thinking about yourself, or replaying the past, a different network hums to life.

That’s the Default Mode Network, or DMN. It includes areas like the medial prefrontal cortex and the posterior cingulate cortex, and it’s responsible for introspection, imagination, and what psychologists call "self-referential thinking."

In moderation, the DMN is part of being human. But when it becomes overactive, it can lead to rumination, worry, and emotional stuckness—things we often see in depression.

Depression and an Overactive DMN

Research shows that people with major depressive disorder have increased DMN activity, especially in regions associated with negative self-focus. It’s like a feedback loop: the more you ruminate, the more this network stays switched on, and the harder it becomes to break free from those thoughts.

This might explain why depression can feel so mentally “loud”—like your brain won’t give you a moment of silence.

A Hidden Link to Alzheimer’s

Here’s where it gets more concerning: people with a history of depression are at a significantly higher risk of developing Alzheimer’s disease later in life. And it’s not just about mood.

Studies have found that the DMN is a hotspot for the buildup of amyloid-beta plaques, the sticky proteins that collect in the brains of people with Alzheimer’s. In fact, the brain areas most active when we’re "mind-wandering" are the same regions where Alzheimer’s tends to hit first.

Some scientists believe that chronic overuse of the DMN may accelerate plaque formation over time.

When Brain Activity Fuels Inflammation

The story doesn’t end there. Overactive brain networks don’t just cause mental noise—they can also trigger inflammation.

Think of microglia as the brain’s housekeepers. They clean up damaged cells and remove excess proteins like amyloid-beta. But when the DMN is constantly firing, the environment becomes inflamed, and these cleanup cells stop working properly.

Over time, this breakdown in maintenance can lead to plaque accumulation, neural damage, and the cognitive decline we see in Alzheimer’s.

Can Brainwaves Help Reverse It?

One of the most fascinating discoveries in recent years came from researchers at MIT. In a 2016 study, scientists exposed mice to flickering LED lights at 40 hertz—a rhythm that mimics a natural brainwave called gamma frequency. The result? A dramatic reduction in amyloid plaques in the visual cortex.

The lights appeared to "entrain" the brain’s rhythms and reactivate microglia, helping them clear harmful buildup. While this doesn’t directly target the DMN, it shows that specific brainwave frequencies can influence immune function and potentially slow or reverse neurodegenerative changes.

It’s still early, but this opens the door to non-invasive therapies that might one day help modulate the DMN and protect the brain.

The Takeaway: Still a Mystery, But a Crucial Clue

The relationship between the DMN, depression, and Alzheimer’s is complex but compelling. What we do know is that:

• Overthinking isn’t just a mental habit—it’s a neurological state.

• An overactive DMN may wear down the brain’s defenses over time.

• Chronic depression could leave the brain more vulnerable to later-life dementia.

If you’ve struggled with depression or persistent mental chatter, this research isn’t meant to alarm—it’s meant to empower. Understanding how the brain works gives us better tools for healing, prevention, and hope.

Sources (for the curious)

• Buckner et al., 2008 – The brain’s default network: Anatomy, function, and relevance to disease

• Greicius et al., 2004 – Default-mode network activity distinguishes Alzheimer's disease from healthy aging

• Iaccarino et al., 2016 – Gamma frequency entrainment attenuates amyloid load and modifies microglia

• Sheline et al., 2010 – Resting-state functional MRI in depression

• Heneka et al., 2015 – Neuroinflammation in Alzheimer’s disease

• Ownby et al., 2006 – Depression and risk for Alzheimer disease: A meta-analysis