How to disable/enable the sleep mode programmatically in iOS?

I have an app that must stay awake until the end of a countdown but it will go into 'sleep mode' whenever it reaches the allocated time to sleep.

In my app, I have the option to postpone sleep, so users can disable/enable it.

How do I do it programmatically?

You can disable the idle timer as follows;

In Objective-C:

[UIApplication sharedApplication].idleTimerDisabled = YES;

In Swift:

UIApplication.sharedApplication().idleTimerDisabled = true

In Swift 3.0 & Swift 4.0:

UIApplication.shared.isIdleTimerDisabled = true

Set it back to NO or false to re-enable sleep mode.

For example, if you need it until you leave the view you can set it back by overriding the viewWillDisappear:

override func viewWillDisappear(_ animated: Bool) {
    UIApplication.shared.isIdleTimerDisabled = false
}

More about UIApplication Class.

In Swift 3, to disable the idle timer it is now:

UIApplication.shared.isIdleTimerDisabled = true

To turn the idle timer back on it is simply:

UIApplication.shared.isIdleTimerDisabled = false

Additionally, note that YES and NO are not available in Swift and that you must use either true or false (as opposed to the previous answer).

iOS 13, Swift 5,5.1+ to disable the idle timer. In SceneDelegate.swift.

 func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
     UIApplication.shared.isIdleTimerDisabled = true
 }

in Swift 3 exact location where this can be done is AppDelegate.swift - you should add UIApplication.shared.isIdleTimerDisabled = true inside application func so result will look like this:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    UIApplication.shared.isIdleTimerDisabled = true
    return true
}