1 May 2026 · Axite

Why You Can’t Focus for More Than 90 Minutes (Everything About Ultradian Rhythms)

What I learned this week working in a Neuroscience startup, as a non-neuroscientist

When I come in at work in the morning, I always start off great. You know that feeling? When you are absolutely flying through your work. You have the feeling you can finish your whole to do list, and everyone else’s? And then, almost without warning, you can’t string a sentence together if your life depended on it. When this happens to me I always grab a coffee and get distracted by something on my phone or talking to my colleagues. Then I, reluctantly, go back to work.

But then somehow… I’m fine again!

Sometimes I try to suppress the feeling and continue working, but it turns out: the slump does not respond well to being ignored.

I used to think that was just me being bad at focusing. But it turns out, it might not be so strange at all! It’s just the brain running its natural cycle. The error in your brain is actually right on schedule. Naturally, when something has to do with the brain, I try to investigate it. Maybe I’ll even impress some colleagues? So I got digging.

This week’s rabbit hole: ultradian rhythms.

So What Even Is an Ultradian Rhythm?

This word sounds like it’s from another planet, filled with aliens trying to attack us, but it actually just stands for any biological cycle shorter than 24 hours that repeats multiple times a day. You might have heard of circadian rhythms (I don’t blame you if you haven’t), the 24-hour internal clock that tells you when to sleep and when to wake up. Ultradian rhythms are the smaller siblings. They run along in the background every 90 to 120 minutes, all day long. Just like my real siblings used to do, actually.

This isn’t just some made-up cycle, but was first described by a sleep researcher named Nathaniel Kleitman in the 1960s. He is also the person who discovered REM sleep. He noticed that the 90-minute cycles he was seeing in sleeping brains did not simply stop when people woke up. The rhythm kept going. It just looked different during the day.

He called it the Basic Rest-Activity Cycle, or BRAC. Which is a very serious name for something that basically explains why you cannot focus for 8 hours straight.

Forward this to the colleague who is always pushing through the 3pm slump. They’ll thank you for it!

What Actually Happens Every 90 Minutes?

The good part: your brain is genuinely switched on

At the start of each cycle, your brain is full of energy. You are ready to be alert, react sharp and take on complex things. This is the window where you feel like the smartest person in the room, coming up with new ideas and fixing everyone’s problems. You might even, if you do say so yourself, be employee of the month! You are riding on those peaks.

The confusing part: and then it stops

Around the 80 to 90 minute mark, something shifts. Your thoughts start to drift, you can’t really focus on that difficult task your boss gave you. You reread the same email over and over again, without retaining any of it. You might even start to yawn. What happened to that smart, insightful employee of the month?

But let me tell you, this is not you lacking. This is just your brain filing a formal request for a break.

It has been processing at full tilt and it now needs a few minutes to consolidate what it has learned, do a bit of internal housekeeping, and prepare for the next round. A part of the brain called the default mode network becomes more active during this window. It is the system associated with daydreaming, reflection and making unexpected connections between ideas. Your brain is not shutting down. It is just preparing you for your next round of focussing.

The problem is that most of us interpret this signal as failure and either push harder or reach for caffeine. Which is a bit like ignoring your car’s fuel light and hoping it sorts itself out.

The part most people skip: actually resting

A mandatory rest? Yes, please! A genuine rest for about 10 minutes, where you are not scrolling, not consuming and not making decisions, allows the brain to complete its reset and come back properly for the next cycle. Most people don’t seem to do this, but it could actually really improve your productivity for the day.

Things you can do to unwind your brain:

Short walk outside
Just sitting quietly
Deliberate daydreaming
Enjoying the sun
Talking to a colleague

The key word is: genuine. No consuming! Switching from your work tab to LinkedIn or Instagram is not rest. Your brain knows the difference, even if you are trying to convince it otherwise (this is also a reminder for myself…).

Can you calculate the cycle?

The simple anwser is: no. Here is what makes this cycle thing genuinely tricky: it does not come with a notification. It’s not like my Google Calendar sending me a reminder when a meeting is coming up. I have to figure out myself when I need to take a small break. But the calculation isn’t easy at all. The exact length of each cycle shifts based on how well you slept, how stressed you are, whether you have been moving your body, and a dozen other factors.

And the worst part: once you start to notice the crash, it’s already too late. You don’t notice it when it’s building, but when you’ve already been fighting it for a while. Taking a break at the right time is a really specific skill, where you have to actively listen to your brain and body.

This uncertainty about the brain is exactly what excites me about working in a company that thinks about solutions for the invisibility of the brain. I’m thinking: maybe we could one day actually read this cycle, just from behind our desk. Thanks to remote brain monitoring hardware and the software we’re developing, this might not be such a crazy thought.

So, what can you do to work with your brain instead of against it?

Understandably, you are not able to measure the cycle from home just yet. I think we can start with just paying better attention to the rhythm itself, instead of suppressing it. Push through the dips, rely on caffeine, ignore the rest signals, and you are not getting more out of your brain. You are borrowing against the next cycle. The debt shows up by mid-afternoon. But if you work with your brain, you can actually be productive for most of the day!

I’m not an expert, but here’s a list of things that I’ve been trying to do since learning about the cycle.

Notice the dip before you crash into it: Once you know the cycle exists and roughly how long it runs, you start catching the early signals. The first yawn, the first wandering thought. Take that break! You will love yourself for it later in the day.
Build breaks in before you need them: Instead of working until you grind to a halt and then collapsing, try intentionally stepping away around the 80 to 90 minute mark. Even ten minutes of genuine rest makes a noticeable difference to the quality of what comes after.
Stop treating the dip as a personal failing: This one is underrated. A lot of guilt lives in that mid-morning or mid-afternoon slump. The vague sense that more disciplined people do not feel this way. They do. Their brains are also running on a 90-minute clock. They just respect the slump and have found ways to relax their brain.

It’s so interesting to think about all the things our body is doing, without us knowing about it or being able to control it. The brain is running these elaborate cycles, sending signals, switching modes, doing maintenance, and we are just up here on the surface making to-do lists and wondering why we feel so exhausted at 3pm. It’s our own choice whether we work with our brain, or against it.

I find that equal parts humbling and fascinating. But mostly fascinating. Which is probably what happens when you spend enough time around people who think about brains for a living.