Swap Forest's trees for Jeju haenyeo: a small Korean team built a study timer in 6 weeks

A two-year-revenue-zero SaaS startup in Seoul switched to "build one small app every 6 weeks" and made 숨비 (Sumbi): put your phone down and you become a haenyeo diving, and the longer you dive, the more likely you find deep-sea creatures in the shells. A month and a half after launch, 54 ratings, 4.98 stars. It caught the college entrance exam season but is still far from breaking out. This covers what it did right, where it got stuck, and whether copying it in another country is worth it.

A student during exam week

In 2026 Oct 21, a Korean student left a five-star App Store review for 숨비 titled "Thanks to 숨비, this exam week was fun":

"After I installed 숨비, I stayed at my desk longer... When I see the cute fish, I want to see more. I've recommended it to friends and we're all using it. Can you let a partner dive together? A few of my friends have been secretly studying behind my back ㅜㅜ" (App Store Korean review, 10-21)

At the end she added: "I'm going to Jeju Island next week, hehe."

A later version did add "함께 잠수" (dive together): you and a partner dive in the same sea, and on screen your haenyeo and theirs sink side by side. The website describes it this way: "This isn't surveillance, it's a diving companion."

What it is

A focus timer. Pick a duration, then pick a 0 to 60 second "breather" (breath-hold time, during which you can briefly switch to other apps), then put your phone down. When time is up the haenyeo surfaces, and a shell on the seabed holds a sea creature: a clownfish from the shallows, a green sea turtle from deeper water, and the "aurora jellyfish" from the deepest zone. Creatures move into "My Sea," and you get schools of them when you find the same species again.

The name comes from the whistle the Jeju haenyeo blows when surfacing to breathe, "숨비소리" (sumbisori). The first line of the store description is "폰을 내려놓은 시간만큼, 삶은 깊어집니다" (The longer you put your phone down, the deeper your life becomes).

Anyone who knows Forest recognizes it at once: leave the phone and a tree grows, leave mid-session and the tree dies. 숨비 swapped trees for sea and divers, and softened the rules:

• No phone lock. One user said it bluntly in a review: "Lock apps on iPhone don't work, you just turn them off in Settings. This one only needs to stay open, and when it ends you get cute fish." (10-26)
• No penalties for failing. From the website: "No trees that die, no red warnings. Only a line that says 'you surfaced a little early.'"
• Calls don't interrupt. Take the call, come back, and the dive keeps counting.
• No gacha odds, no subscription, no ads. The top deep-sea creature chance of 8% is printed right in the app; "you can't buy better odds." The basic features are free, and if you like a creature you can "invite" it for a small one-time purchase.

Who makes it

The developer is나이스데이 (niceday), who previously built enterprise organizational culture SaaS. The CEO wrote about it on the company blog (2026-09-28): the product launched 2024 years ago in September, and "revenue to date is 0." As AI sped up development and the SaaS market cooled, they changed approach in 2026 April: every idea goes through three questions first (how big is the market, why do users stay, will they pay), and if a first version can ship in 6 weeks, they build it. 2 weeks to think it through, 2 weeks to build, 2 weeks to ship.

숨비 is the second app on this pipeline. The first was "킁킁", a dog-walking social app launched in August, which has only 7 ratings on the store.

How big it is now

Not very big.

At 30 to 60 minutes per dive, 900k minutes is roughly 15k to 30k dives, so combined across both platforms a few thousand users (estimate). The bar for Korea's productivity free chart around #80 is low, a few hundred downloads a day can get you there. It hasn't made Top Grossing, so in-app purchase revenue is likely small.

Why it has some traction

The theme makes people want to stay. Reviews keep saying the same thing: "So cute", "I can't resist when a new ocean friend shows up", "Collecting them one by one is my recent small joy." A focus timer is nothing new, and the anticipation of collecting is what keeps people running it, the same psychology as gacha. Some people were let down too: "I focused for a long time to see the deep-sea creatures, and got shallow-water ones, a bit deflating" (11-04).

Good timing. Korean middle-school midterms are in October, and the college entrance exam is in Nov 19. Reviews include "I'm a senior preparing for the exam" (10-04), "exam week" (10-21), and on Play, "Using it constantly because it's exam week."

Bought some Instagram ads. One Google Play review reads: "First off, this isn't an ad. The quality is clean and cute, no ads, you can't spend money on gacha... I installed it from an Instagram ad, it worked so well I came straight to write a review." The website homepage features this one. How much was spent and what the creatives looked like isn't visible from outside.

Friends pulling friends. Partner dives, sending a QR code that gives a creature, diving together: these are features that only work if you bring people in. "Recommended it to my friends" shows up more than once in reviews.

The store title borrows from Forest. 숨비's subtitle is "공부 집중 타이머 & 디지털 디톡스", and Forest's Korean title is "집중 공부 타이머 & 디지털 디톡스". Only the first two words are swapped. Plenty of people search "공부 타이머" in exam season, and it's catching some of that traffic.

No coverage turned up in media, review blogs, TikTok or X.

Who it's competing with

Two things are worth remembering. First, in Korea's category, everything that makes money is a subscription: FocusFlight, 잠글시간 and 터닝 are all on Top Grossing, while 숨비 earns from one-time purchases of single creatures and hasn't charted. Second, I didn't find anyone else running a focus timer built on haenyeo (Jeju sea divers) or marine life with real volume in Korea. This theme is still open.

Where it's stuck

Distribution. The product reputation is fine, 4.98 stars, and the developer replies to every review. But an Instagram account with 70 followers, a little ad spend and a low-barrier category chart don't add up to volume.

Money. "No subscription" is both its selling point and its problem. One reviewer ran the numbers: "Unlocking all the deep-sea creatures costs about 50k won, that's a burden. Why not make it a monthly fee that unlocks everything while subscribed?" (10-24) The developer replied they'd consider it. 11-05 Another five-star review opens with "it's a bit of a shame to have to pay." The business model isn't settled.

Seasonality. Whether demand from students drops off once the 11-19 College Scholastic Ability Test is over, I can't tell yet.

Small UX issues. It breaks if you switch out to look up a word or use GPT as a timer (11-03). Users don't understand how "breathing time" is calculated (11-04). There's no end-of-session sound (10-30, 10-31).

If it were you

C

You can learn a cheap way to play here, but 숨비 itself hasn't yet proven this can scale or earn money. If you copy it as-is, you'll most likely also end up with a few thousand users.

Who it suits: a small app team of 1 to 3 people, with an artist who can draw cute characters, and a cultural symbol from your home country that locals instantly recognize. The target country should have a concentrated exam season (Japan's January common test, China's June gaokao, Taiwan's January exam).

Who it doesn't suit: teams without an artist (these apps rely entirely on cute characters); anyone wanting the Korean market (숨비 already owns haenyeo, and 열품타 is above it); anyone unwilling to do subscriptions (every app in this category that charts on Top Grossing is a subscription).

How to build it: Forest's skeleton with a local theme, with soft rules: no phone locking, no penalty for failing, incoming calls don't break the session. Collection is tiered shallow, medium and deep, with the odds made public, and copy the 11-04 review pattern so that people who dive long enough always get a rare one (숨비's randomness is discouraging). Start with a subscription from day one and unlock everything while subscribed. Don't wait for users to ask. Build partner and diving-together features into v1; that's the only free acquisition channel.

How to acquire users: start running Instagram and TikTok ads two months before the exam, with creative showing a phone face-down on a desk and a character slowly sinking on the screen. Write store keywords against the top local study-timer apps. In the week after launch, get a batch of classmates and friends to add each other as partners.

Timing: to make Japan's January 2027 common test, you need to launch before early December, and working back at 6 weeks, you have to start now. If you miss it, the next window is the June gaokao.

Signs you've misread this: (1) For two or three weeks after the College Scholastic Ability Test, 숨비's rating growth ran below the 1 to 3 per day it saw in October, which would mean this kind of product only lives on exam season and doesn't retain users. (2) After 숨비 moved to a subscription, it still couldn't crack the top 200 on the Korean Productivity Top Grossing chart, which would mean cute themes don't drive payment. (3) Conversely, if it charts on Top Grossing in December, this path is better than it looks now, and the grade should be raised.