Timers trigger more timers.

App.js

takePicture = () => {
  this.camera.takePictureAsync({
    quality: 0.1,
    base64: true,
    exif: false
  }).then(photo => {
    this.setState({ photo });

    // In 27 seconds, turn the camera back on
    setTimeout(() => {
      this.setState({ photo: null });
    }, PHOTO_INTERVAL - FOCUS_TIME);

    // In 30 seconds, take the next picture
    setTimeout(this.takePicture, PHOTO_INTERVAL);
  });
}