# Soln — Sleep Engine

> Sleep Engine (iPhone/iPad app by Nan Yu.)

- Source: https://appshunter.io/ios/app/soln/id6772215173 (this page in markdown: same URL + `.md`)
- Developer: [Nan Yu](https://appshunter.io/developer/1559044036)
- Category: Health & Fitness, Productivity
- Price: Free with in-app purchases
- Age rating: 4+
- Requires: iOS 17.0 · 3 MB
- Languages: American English, Dutch, French (France), German (Germany), Italian, Japanese, Korean, Brazilian Portuguese, Russian, Chinese (Simplified, China) and 2 more
- Released: 2026-05-26
- Data updated: 2026-09-09
- User reviews in markdown: https://appshunter.io/ios/app/soln/id6772215173/reviews.md

## What is Soln?

Soln is a sleep engine built on cognitive load theory.

Different from other sleep apps

Most sleep apps are just players — they start audio and leave you to it. Soln calculates how hard it will be to fall asleep tonight, decides which layer to start at and how long to stay there, then stops when you're out. You don't have to manage anything.

Five audio layers

· Silent preparation — nothing playing, just space for your mind to settle
· Minimal white noise — a steady background to mask outside noise
· Heart rate-synced white noise — audio tempo follows your heart rate; as it slows, so does the sound
· Tonal fade — frequency gradually drops, sliding your brain from alert to relaxed
· Semantic drift — soft, loosely connected speech that gives your mind something to dissolve into instead of looping the same thought (paid)

How long you spend on each layer isn't fixed — it depends on how you're doing tonight.

Apple Watch auto-stop

When you fall asleep, Apple Watch detects it. Soln fades the audio out over two seconds, then stops. No timer to set. No sudden silence to wake you (paid).

Nightly assessment

Before bed, based on today's mood and last night's sleep, Soln estimates how hard it'll be to fall asleep and gives you an optimal bedtime window and expected sleep onset time.

In the morning, three scores: reaction, memory, and mood. Sleep well, score well. Sleep poorly, score reflects that. Straightforward.

Full sleep history

A three-color ring for each night. Long-press any day for that night's detail and sleep-onset timeline.

One-time purchase

Free for 7 days, full features included. After that, a one-time purchase permanently unlocks everything. Tied to your Apple ID — restore on any new device. No ongoing charges.

## Pricing and in-app purchases

Base price: Free.

**One-time purchases**

| Purchase | Price | Type |
| --- | --- | --- |
| Unlock Soln | $5.99 | one-time |



## Version history (last 3 releases)

### 1.0.2 — 2026-07-01

• If you're still awake after the sleep engine finishes, a gentle prompt now appears — choose to try again or simply end for the night
• Fixed an issue where the optimal sleep window sometimes showed the current time instead of your target bedtime
• Fixed a bug where late-night sleep sessions occasionally appeared under the wrong date in the history calendar

### 1.0.1 — 2026-06-08

• Science library rebuilt — 19 curated peer-reviewed studies, including foundational research on white noise, pink noise, and closed-loop auditory stimulation
• Clearer gesture hints in the Sleep Engine
• Bug fixes and performance improvements

### 1.0.0 — 2026-05-26

No release notes.

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## More apps by Nan Yu

- [浑水摸鱼](https://appshunter.io/ios/app/id1362935586)
- [Renobo: Ebook & Manga Reader](https://appshunter.io/ios/app/renobo-ebook-and-manga-reader/id1575881378)
- [Cafactor](https://appshunter.io/ios/app/cafactor/id6757445668)
- [Post Rush](https://appshunter.io/ios/app/post-rush/id6757679826)
- [Floor 73](https://appshunter.io/ios/app/floor-73/id6759790029)
- [CíBuch 中德词簿](https://appshunter.io/ios/app/cibuch/id6761343922)

All apps by Nan Yu: https://appshunter.io/developer/1559044036

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*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-09-09.*