Mental Fatigue Index (MFI): a 0–100 “brain battery” check

Mental fatigue is that specific kind of tired where your body might be able to move, but your mind feels slow, foggy, irritable, or “over it.” You read the same sentence three times. Small tasks feel heavy. Your attention keeps sliding off whatever you’re trying to do. That’s mental fatigue — and it tends to spike when a few common drivers stack up: sleep debt, long focus blocks, constant screen exposure, high stress, and low recovery.

This calculator turns those drivers into one simple score: 0 = fully fresh and 100 = maxed-out mental fatigue. It’s not a diagnosis. It’s a practical snapshot you can use to decide whether you should push through, take a strategic break, or call it early and recover.

• 0–24 (Fresh): clear head, stable focus, low irritability. Great day for deep work.
• 25–49 (Loaded): you can function, but attention is “slippery.” Schedule breaks.
• 50–74 (Drained): errors rise, motivation drops, scrolling becomes tempting. Reset needed.
• 75–100 (Overheated): brain fog + low tolerance + poor decisions. Recover first.

If fatigue is severe, persistent, or paired with alarming symptoms (chest pain, fainting, severe depression, thoughts of self-harm, etc.), seek professional support.

How the score is calculated

The Mental Fatigue Index is a weighted blend of eight signals. Each input becomes a sub-score from 0–100 (higher = more fatigue), then we combine them with weights that reflect how strongly each factor usually affects focus and cognitive endurance.

• Sleep debt signal: compares your sleep to an “ideal” target (default 8h). Less sleep → higher fatigue.
• Focus load signal: long uninterrupted focus hours are powerful, but they cost mental energy.
• Break deficit signal: fewer breaks than recommended increases fatigue; regular breaks lower it.
• Screen time signal: more screen hours tends to correlate with attention fatigue and poor sleep quality.
• Stress signal: higher stress raises cognitive load and makes recovery harder.
• Hydration deficit signal: dehydration often shows up as brain fog and low energy.
• Exercise deficit signal: movement supports mood, sleep quality, and alertness; none can worsen fatigue.
• Late caffeine signal: caffeine too late can disrupt sleep and next-day clarity (even if you “feel fine”).

We compute a weighted average. Sleep and stress have slightly higher influence because they often dominate how the brain feels. Breaks and focus load are next (they’re the “work structure” levers). Screen time, hydration, exercise, and late caffeine round it out.

• Sleep: 22%
• Stress: 18%
• Focus load: 16%
• Break deficit: 14%
• Screen time: 12%
• Hydration deficit: 8%
• Exercise deficit: 6%
• Late caffeine: 4%

Result: a single number from 0 to 100. You’ll also get a breakdown so you can see what’s driving the score — which makes it easy to create a small, realistic plan (instead of vague “I should do better” feelings).

Three real-world scenarios

Example 1: “I slept well but I’m still tired.”
Sleep 8.0h, stress 6/10, focus 6h, breaks 1, screen time 7h, water 5 cups, exercise 0, caffeine after 4pm: yes. Your score might land around the 40–55 range: the hidden culprits are stress + long focus + screens + late caffeine. Even with good sleep, your brain can still feel taxed if recovery signals are missing.

Example 2: “Deadline week: I’m fried.”
Sleep 5.5h, stress 9/10, focus 9h, breaks 0, screen time 11h, water 3 cups, exercise 0, caffeine after 4pm: yes. This typically lands in the 80–95 range. The calculator will push you toward recovery actions: protect sleep, add micro-breaks, reduce late caffeine, and schedule a reset block.

Example 3: “Pretty balanced day.”
Sleep 7.5h, stress 3/10, focus 4h, breaks 3, screen time 4h, water 8 cups, exercise 30 minutes, caffeine after 4pm: no. This usually produces a 10–25 score — a “green-light” day for deep work.

• They show you how tradeoffs work: good sleep can be canceled by stress + screens + no breaks.
• They show you what to change first: the biggest levers are usually sleep, stress, and break structure.
• They make the score feel “real”: you can compare your day to a similar scenario.

Use it as a “screenshot score” for your daily reset

The fastest way to get value from this tool is to treat it like a daily check-in: run it once mid-day (after your biggest work block) and once in the evening. If the score is trending upward, you can intervene early — before your attention collapses into doom-scrolling or low-quality work.

• Late morning: confirm you’re actually fresh before scheduling deep work.
• Mid-afternoon: see whether you need a real break vs more caffeine.
• Evening: decide whether to recover (sleep + walk) or squeeze one more task.

• Run “what-if” tests: add one extra break, reduce screen time by 2 hours, or move caffeine earlier. Watch the score change.
• Share your score: “My MFI is 72… I need a reset” is a surprisingly relatable post.
• Make it a team ritual: a quick MFI check before a long meeting helps people pace themselves.

If you want a simple rule: when you cross 50, switch from “push” mode to “protect” mode. Protect your attention like a budget — because it is.

Ranges + what to do next

Your score isn’t a moral judgment. It’s a signal. Here’s a practical playbook for each range:

• 0–24 (Fresh): Schedule your hardest task now. Use 50/10 or 60/10 work blocks.
• 25–49 (Loaded): Do 25–40 minute sprints. Take real breaks. Hydrate.
• 50–74 (Drained): Shift to admin tasks, planning, or short meetings. Add movement and recovery.
• 75–100 (Overheated): Protect sleep, reduce screens, lower stress load, and reset your nervous system.

• Stand up + deep breathing (60 seconds).
• Water (1–2 cups).
• Look far away / sunlight (60 seconds).
• Quick stretch (90 seconds).
• Write the next 1 task only (60 seconds).

Frequently Asked Questions

• Is this a medical test?

  No. It’s a wellness-style index to help you notice patterns. If you’re worried about chronic fatigue, consult a clinician.

• Why is sleep weighted so heavily?

  Sleep is the biggest input into attention, mood stability, and cognitive speed. Even small sleep debt can add up fast.

• What counts as a “break”?

  A real break is a short off-screen reset: stand up, breathe, walk, stretch, water. A “scroll break” often doesn’t reduce fatigue.

• Can caffeine lower the score?

  This index focuses on fatigue drivers, not stimulants. Caffeine might mask tiredness short-term but can worsen sleep if taken late.

• How can I lower my MFI quickly?

  Try: 10-minute walk + water + 5 minutes of eyes-closed breathing + a snack with protein. Then do a 25-minute focused sprint.

• What’s a “good” score for deep work?

  Ideally under 35. If you’re 35–50, do shorter blocks and more breaks. Over 50: do lighter tasks or recover first.

• Why do screens matter if I’m productive?

  Screen time can increase eye strain, reduce blink rate, and push bedtime later — all of which can raise mental fatigue.

More tools that pair with Mental Fatigue

If you’re optimizing focus and energy, these tools stack perfectly with your MFI score. Pick one and build a small routine.