docs: 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