def current_sleep_breakdown(self):
        """Return durations of sleep stages for in-progress session."""
        try:
            stages = self.intervals[0]['stages']
            breakdown = {'awake': 0, 'light': 0, 'deep': 0, 'rem': 0}
            for stage in stages:
                if stage['stage'] == 'awake':
                    breakdown['awake'] += stage['duration']
                elif stage['stage'] == 'light':
                    breakdown['light'] += stage['duration']
                elif stage['stage'] == 'deep':
                    breakdown['deep'] += stage['duration']
                elif stage['stage'] == 'rem':
                    breakdown['rem'] += stage['duration']
        except KeyError:
            breakdown = None
        return breakdown
Return durations of sleep stages for in-progress session.