After all, every single thing you experience,

your thoughts and your actions,

your perceptions and your memories

are processed here in your body's control center.

But if this already seems like a lot for a single organ to handle,

it's actually only a small part of what the brain does.

Most of its activities are ones you'd never be aware of,

unless they suddenly stopped.

The brain is made up of billions of neurons,

and trillions of connections.

Neurons can be activated by specific stimuli or thoughts,

but they are also often spontaneously active.

Some fire cyclically in a set pattern.

Others fire rapidly in short bursts before switching off,

or remain quiet for long periods

until thousands of inputs from other neurons line up in just the right way.

On a large scale,

this results in elaborate rhythms of internally generated brain activity,

humming quietly in the background

whether we're awake, asleep,

or trying not to think about anything at all.

And these spontaneously occurring brain functions

form the foundation upon which all other brain functions rely.

The most crucial of these automatically occurring activities

are the ones that keep us alive.

For example, while you've been paying attention to this video

spontaneous activity in your brain has been maintaining your breathing

at 12 to 16 breaths a minute, making sure that you don't suffocate.

Without any conscious effort,

signals from parts of your brainstem are sent through the spinal cord

to the muscles that inflate your lungs,

making them expand and contract, whether or not you're paying attention.

The neuronal circuits underlying such rhythmic spontaneous activity

are called central pattern generators,

and control many simple repetitive behaviors,

like breathing,

walking,

and swallowing.

Ongoing neural activity also underlies our sensory perception.

It may seem

that the neurons in your retina that translate light into neural signals

would remain quiet in the dark,

but in fact,

the retinal ganglion cells that communicate with the brain

are always active.

And the signals they send are increases and decreases in the rate of activity,

rather than separate bursts.

So at every level, our nervous system is teeming with spontaneous activity

that helps it interpret and respond to any signals it might receive.

And our brain's autopilot isn't just limited to our basic biological functions.

Have you ever been on the way home,

started thinking about what's for dinner,

and then realized you don't remember walking for the past five minutes?

While we don't understand all the details,

we do know that the ongoing activity in multiple parts of your brain

is somehow able to coordinate what is actually a complex task

involving both cognitive and motor functions,

guiding you down the right path and moving your legs

while you're getting dinner figured out.

But perhaps the most interesting thing about spontaneous brain function

is its involvement in one of the most mysterious

and poorly understood phenomena of our bodies: sleep.

You may shut down and become inactive at night,

but your brain doesn't.

While you sleep,

ongoing spontaneous activity gradually becomes more and more synchronized,

eventually developing into large, rhythmic neural oscillations

that envelop your brain.

This transition to the more organized rhythms of sleep

starts with small clusters of neurons tucked in the hypothalamus.

Despite their small number,

these neurons have a huge effect

in turning off brainstem regions that normally keep you awake and alert,

letting other parts, like the cortex and thalamus,

slowly slip into their own default rhythms.

The deeper we fall into sleep,

the slower and more synchronized this rhythm becomes,

with the deepest stages dominated by large amplitude, low frequency delta waves.

But surprisingly, in the middle of this slow wave sleep,

the brain's synchronized spontaneous activity

repeatedly transitions into the sort of varied bursts

that occur when we're wide awake.

This is the sleep stage known as REM sleep,

where our eyes move rapidly back and forth as we dream.

Neuroscientists are still trying to answer many fundamental questions about sleep,

such as its role in rejuvenating cognitive capacity,

cellular homeostasis,

and strengthening memory.

And more broadly, they are exploring

how it is that brain can accomplish such important and complex tasks,

such as driving, or even breathing, without our awareness.

But for now, until we are better able

to understand the inner workings of their spontaneous functioning,

we need to give our brains credit for being much smarter

than we ourselves are.