When comparing I Can't Wake Up! vs Sleep as Android, the Slant community recommends I Can't Wake Up! for most people. In the question“What are the best alarm clock apps for Android?” I Can't Wake Up! is ranked 2nd while Sleep as Android is ranked 3rd. The most important reason people chose I Can't Wake Up! is:
Every feature, especially the tasks, can be customized to a great extent so that it works in the best way for you needs.
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Pros

Pro Highly customizable
Every feature, especially the tasks, can be customized to a great extent so that it works in the best way for you needs.
Pro Awake test
This feature is a fallback in case the user turns off their alarm and goes right back to sleep. The awake test is an optional second alarm that will go off to ensure the user is either already up or to reawaken them. If the user fails to turn off this alarm after a set period of time, the primary alarm (and all of the possible wake up task(s)) goes off again.
Pro Most wake up tasks
8 available wake up tasks can be enabled. Yes, upon waking up, you can force yourself to complete 8 tasks before the alarm can be turned off. These tasks can be customized. The difficulty levels for some of the tasks can be altered depending on how grueling you want your wake up to be. It is even possible to set the alarm to have a random number of tasks.
Pro 'After finish' application
Want an app to automatically open up after the alarm is turned off? This makes it possible.
Pro General settings
Just like most alarm clock apps, this one provides the options to set appearance, notifications, alarm screen, smooth wake ups, time format, and custom alarm tones.
Pro Monitors sounds during sleep
This app will notify users if they are snoring, talking in their sleep, or have sleep apnea.
Pro Sends a notification for sleep deficit
If a user isn't getting enough sleep, this app will send a notification and help them get back on track.
Pro Charts for optimal sleep duration and fall asleep hour
Provides solid suggestions for when to sleep in order to optimize ones sleep quality. It adapts to your sleep patterns over time and relies on ratings provided by the user as well as Deep Sleep % calculated by the app.
Pro Gentle sounds for waking up and falling asleep
A set of nature sounds are included for falling asleep and waking up peacefully. If the sounds aren't enough to wake up, music can be played, or a puzzle alarm can be set.
Pro Smart watch compatibility
This app is compatible with Pebble, Android Wear technology, and Tizen. And some of Garmin wearable devices as well.
Pro Anti-snore monitor
An alarm can be set to vibrate each time a snore occurs.
Pro Powerful alarms
Alarms with "smart period," 8 kinds of captcha, "backup alarm," etc.
Cons

Con Ads
The app does have some banner ads. However, they do not get in the way nor are they abundant.
Con Plain interface
It uses the default white text on black background formatting. The entire app has a plain layout with nothing aesthetically pleasing. Sometimes simplicity is best.
Con Have to manually tell it when you go to sleep
This doesn't work as seamlessly as sleep trackers in wearables, like Fitbit trackers. It doesn't record your sleep time, you have input when you go to sleep and when you wake up, which makes it far less useful.
Con Requires outside file viewer to wake up to music
In order to choose a song on the SD card, a file viewer is required.
