What Actually Affects Your Mood? A Simple 30-Day Experiment to Find Out

Posted on: December 16, 2025

You wake up feeling heavy. Not sick, not sad exactly—just... off.

You try to figure out why. Did you sleep badly? Was it something that happened yesterday? The weather? You scan your memory, but nothing stands out.

By evening, the feeling has passed. You never find out what caused it.

This happens again. And again. Some days feel light; others feel like you're dragging yourself through mud. And you have no idea why.

Here's the thing: most people don't know what actually affects their mood. We have theories—"I'm a morning person," "Mondays are always hard," "Exercise helps me"—but these are guesses, not facts. Without data, we're just speculating about ourselves.

What this article will show you:

Common factors that may influence mood (and why general advice often doesn't apply to you)
Why your memory is unreliable for understanding mood patterns
A simple 30-day experiment to discover what actually affects YOUR mood—no complex tracking required

Factors Worth Investigating

Before you start tracking, it helps to know what to pay attention to. Here are factors that many people find relevant—though whether they matter for you is exactly what the experiment will reveal.

Sleep

How much you slept, when you went to bed, whether you woke up during the night. Some people need 8 hours; others function on 6. Some are sensitive to timing; others to duration. You won't know your pattern until you see your own data.

Physical Activity

Did you exercise? Walk? Sit at a desk all day? Some people feel better after moving; others feel drained. The effect might show up the same day or the next.

Weather

Gray skies, rain, temperature swings, seasonal changes. Some people notice these; others don't at all.

Time with others can energize or exhaust, depending on the person, the context, and a dozen other variables.

Work and Responsibilities

Deadlines, meetings, unfinished tasks. The specifics vary—some feel worse with too much structure, others with too little.

Food and Caffeine

What you ate, when you ate, how much coffee you had. Effects are often subtle and delayed.

None of these are guaranteed to affect you. That's the point. General advice tells you "sleep matters" or "exercise helps"—but it can't tell you how much they matter for you, or which factors are actually relevant to your life.

The only way to find out is to look at your own data.

Why You Can't Trust Your Memory

If these factors are well-known, why don't we already understand our own patterns?

Because human memory is unreliable when it comes to mood.

We Remember Extremes

When you think back over the past week, what stands out? Probably the really good moments and the really bad ones. The ordinary days blur together.

This means your mental picture of "how you've been feeling" is skewed toward extremes. You might think you've been having a terrible month when actually most days were just... fine.

Bad Days Stick in Memory

Think about last month. What comes to mind first—the frustrating moments or the pleasant ones? For most people, negative experiences are easier to recall.

So when you ask yourself "Why have I been feeling bad lately?", the bad days come to mind more easily—even if okay days were just as common.

We Construct Narratives After the Fact

"I felt bad today because of that meeting."

Maybe. Or maybe you felt bad for unrelated reasons and your brain assigned the meeting as the cause because it needed an explanation.

Without real-time records, you're working with reconstructed memories that may not reflect what actually happened.

The 30-Day Experiment

Here's a simple way to cut through the guessing: track your mood for 30 days.

Not in elaborate detail. Not with lengthy journal entries. Just a quick daily check-in.

What You'll Need

A way to record your mood each day (paper, phone notes, or an app)
10-30 seconds per day
Curiosity about what you'll discover

Step 1: Record Your Mood Daily

Every night before bed, record how you felt overall. Pick the same time each day—consistency matters more than precision.

Use a simple scale:

😄 Great
🙂 Good
😐 Okay
😞 Bad
😢 Terrible

Don't overthink it. Go with your gut feeling. The goal is consistency, not precision.

Example:

Dec 1: 🙂
Dec 2: 😐
Dec 3: 😞
Dec 4: 😐
Dec 5: 🙂
Dec 6: 😄
Dec 7: 😐

That's it. Seven days of data in about 70 seconds total.

Want a simple tool for this? Nikklet lets you tap an emoji and you're done—free, no account setup beyond Google sign-in.

Step 2: Add One Variable (Optional)

Start with mood only. That's enough for the first week.

If you want to go further, add just one variable—the one you most suspect affects your mood. For most people, that's sleep. A simple note like "6hrs" or "slept well" is enough.

Don't track multiple factors at once. More variables means more effort, which means you'll quit. One is plenty.

Step 3: Look for Patterns After 30 Days

At the end of the month, review your data. Ask yourself:

Day of week: Are certain days consistently better or worse?
Sequences: Does a bad day tend to follow certain events (poor sleep, skipped exercise)?
Lag effects: Does something like exercise affect you the same day, or the next?
Trends: Is there an overall upward or downward trend, or is it stable?

Important: How to read your data

You're finding clues, not proving causes. If bad mood often follows poor sleep, that's worth investigating—but 30 days of data can't prove sleep caused the bad mood.
Look for patterns that repeat. One bad Monday means nothing. Four bad Mondays in a row is a signal.
Check for lag. Some effects show up the next day, not the same day. If you exercised Tuesday and felt better Wednesday, that's still a pattern.
Ignore outliers. One unusually bad day after good sleep doesn't disprove a pattern. Look at the overall trend.

What Patterns Might Look Like

After 30 days, you might discover things like:

"Mondays are actually fine—it's Sundays that are hard." Many people assume Monday is their worst day. But when they track, some discover that Sunday evening is when their mood dips—anticipation of the week ahead. (This phenomenon is common enough to have its own name: the Sunday Scaries.)

"My mood follows my sleep almost exactly." You might find that bad days almost always follow nights with less than 6 hours of sleep. This isn't surprising, but seeing it in your own data makes it concrete.

"Exercise helps, but not immediately." Some people find that the mood boost from exercise shows up the next day, not the same day.

"I thought I was doing worse than I actually am." Bad days are easier to remember. Many people discover that their "bad month" had more okay and good days than they recalled.

"Weekends aren't actually better." Some people expect weekends to be mood highlights and are surprised to find they're just... average.

Why 30 Days Matters

Why not just a week?

Weekly patterns need repetition. You need at least 4 Mondays to see if Mondays are really the problem.
Noise cancels out. Random variation is less misleading with more data points.
You start noticing naturally. After a couple of weeks, you'll begin forming hypotheses just from the act of recording.

30 days is long enough to see patterns but short enough to be achievable. It's an experiment, not a lifelong commitment.

What to Do With Your Findings

Let's say you discover that sleep is your primary mood factor. Now what?

You don't have to fix everything. The value of this experiment is awareness, not necessarily behavior change.

Sometimes just knowing "today feels hard because I slept badly" is enough. It reframes the day from "something is wrong with me" to "this is a predictable consequence of poor sleep."

Other times, you might decide to run a follow-up experiment: "What if I prioritize sleep for two weeks and see what happens?"

The point is that your decisions become informed rather than based on guesswork.

Making It Stick

The key to finishing 30 days: pair it with something you already do. "After I brush my teeth, I log my mood." That's it.

You'll forget some days. That's fine—just log the next one. For more tips on building this habit, see our guide on why journaling doesn't work and what to do instead.

Tools for the Experiment

You can do this with:

Paper

A notebook or calendar where you mark each day. Simple and effective.

Phone Notes

A running note where you add a line each day. Accessible everywhere.

A Mood Tracking App

If you want calendar visualization and trend graphs without manual effort, Nikklet is designed for exactly this kind of experiment.

5 emoji moods: Choose in seconds
Calendar view: See your month at a glance
Trend graphs: Spot patterns visually
Optional notes: Add context when you want
No streak pressure: Gaps are fine

Free to use. Google sign-in only (no separate password). Your data is not sold or used for advertising.

Starting Tonight

You don't need to wait until Monday or the first of the month.

Tonight, before you sleep, ask yourself: "How was today?"

Pick the emoji that fits. Write it down somewhere.

That's day one of your 30-day experiment.

In a month, you'll have something most people never get: actual data about what affects your mood. Not theories. Not other people's advice. Your own patterns.

Start tracking with Nikklet — or grab a notebook and begin tonight.

Frequently Asked Questions

Do I need to track every single day?

No. Even with gaps, patterns emerge. The goal is "most days," not "every day without exception." Don't let perfectionism become a reason to quit.

What if my mood is the same every day?

That's useful data too. It might mean external factors don't affect you as much as you thought—or that you need more nuanced tracking to detect smaller variations.

Should I track multiple times per day?

For this experiment, once per day is enough. Tracking more often adds complexity without necessarily adding insight. Start simple.

What if I discover my mood is mostly bad?

First, 30 days of data might reveal that you have more okay days than you realized. We tend to remember bad days more easily, so tracking often shows a more balanced picture.

If you do find a persistent pattern of low mood that concerns you, that's valuable information—and it may be worth discussing with a healthcare professional.

Can I track other things besides mood?

Yes, but start with mood only. Adding too many variables makes the habit harder to maintain. Once mood tracking is established, you can layer in other factors.

This article provides information about self-observation and habit-building. It is not medical advice. If you are experiencing persistent low mood, anxiety, or mental health concerns, please consult a healthcare professional.