Or alert air traffic control, and maybe they can send an alert into the cockpit. But there must be a way to raise an alarm.

My guess is basically nobody cares that much about this rare condition. It's much more likely the pilots are incapacitated rather than asleep, in which case the alarm is pointless.

But I guess it's not that rare:

  - https://www.newsweek.com/pilots-boeing-737-both-doze-off-overshoot-airport-two-hour-flight-1735435
  - https://jalopnik.com/tale-of-both-pilots-sleeping-during-flight-reminds-us-t-1849014190
  - https://www.cnn.com/travel/article/pilots-sleep-cockpit-airline-safety/index.html
  - https://www.bbc.com/news/uk-24296544
  - https://www.cbsnews.com/news/both-pilots-fall-asleep-during-british-airbus-flight-incident-report-reveals/
  - https://www.aero-news.net/index.cfm?do=main.textpost&id=A9AAFE58-18F4-40B5-B59A-EBE316BBE878
  - https://content.time.com/time/nation/article/0,8599,1932040,00.html