Quick Answer: Sleep Cycle is a sleep tracker with a smart alarm: it tries to wake you in light sleep so mornings feel less groggy. ARC is a circadian rhythm app for the rest of the day: your morning light window, your caffeine cutoff and your peak and dip. They solve different problems, and many people could use both. Disclosure: we make ARC.
Sleep Cycle is one of the most downloaded sleep apps in the world. ARC has a fraction of those downloads — but solves a completely different problem. Understanding the difference will tell you exactly which app you actually need.
What Sleep Cycle Does#
Sleep Cycle mainly uses your iPhone's microphone to detect movement and breathing sounds during sleep. It uses this data to identify your sleep phases and wake you during the lightest phase within a 30-minute window before your alarm. The result: you wake up feeling less groggy because you were not yanked out of deep sleep. (You can do a rough version of the same math for free with our sleep cycle calculator.)
It also records sleep quality trends over time, shows you a sleep graph for each night, and has a basic smart alarm analysis feature with AI-based insights.
The Core Limitation of Sleep Cycle#
Sleep Cycle solves the waking up problem. It does not solve the day problem.
After Sleep Cycle wakes you up, you are on your own. When do you get sunlight? When do you have your first coffee? What time is your brain actually sharp enough for your most cognitively demanding work? When does your caffeine cutoff need to happen to protect your deep sleep tonight?
Sleep Cycle isn't built to answer these questions. It is a retrospective tool — it analyzes last night. ARC is a prospective tool — it plans today.
What ARC Does After You Wake Up#
ARC's entire design is built around what happens after you open your eyes.
Log last night's sleep in one tap. If it was short, ARC's sleep-debt recovery loop changes the day's sentence, the one ask and your caffeine cutoff for the next few days.
The light window opens at your real sunrise, and a 20-minute morning light timer keeps running on your Lock Screen and Dynamic Island while you're outside. The caffeine tracker lets you log your first drink and immediately shows you your active mg, your decay curve, and your last safe coffee for tonight's bedtime (the free caffeine calculator shows the same math for a single drink).
If mornings are still rough even after a smart alarm, sleep inertia may be the culprit; see why you wake up tired after 8 hours.
You are not just waking up gently. You are syncing your entire day with your biology.
Who Needs Sleep Cycle#
Sleep Cycle is great for: people who struggle with morning grogginess, those who want a simple sleep quality log, and people who are not ready to make broader lifestyle changes around circadian science.
Who Needs ARC#
ARC is for people who already sleep reasonably well but are not getting the most out of their days — founders, engineers, remote workers, and anyone who has noticed that their energy, focus, and creativity seem to follow a pattern they have not figured out how to control yet.
Can You Use Both?#
Yes. Sleep Cycle for a gentle wake. ARC for everything after that. The two apps do not compete; they complement each other at different points of the circadian cycle.