Is watching the heart beat (instead of watching the breath) a known form of meditation? In Anapana there is no regulation of the breath.


As I understood it, it's meant not to control your breath while meditating, but just let flow. Sometimes it's difficult for me not to control it, so I thought about watching my heart beat. Something I have less control of than my breath.

Is this kind of meditation known?
The only time I've seen the heart-beat mentioned in a similar context, is when Trungpa Rinpoche discussed beginner-level practice:


  ...Usually hinayana-level student works with reality using exclusively phenomenological methods. He pays attention to his heart-beat, he watches his breathing, controls his posture and gait, how he eats, how drinks water etc...


The reason the breath is watched (as was explained by Trungpa Rinpoche) is because breath is the window onto the state of mind, or rather, the state of the entire mind-emotions-body continuum:


  If there is an out-breath conditioned by the mind, he is aware that the out-breath was conditioned by the mind. If there is an in-breath conditioned by the mind, he is aware that the in-breath was conditioned by the mind.
  --from Ānāpānasmṛti Sutra (EA 17.1) 


One you get access to your state of mind by seeing how your breath is "conditioned by the mind", you use the feedback of the breath to work on calming and gladdening the entire body-mind:


  ’Breathing in, I am aware of my whole body. Breathing out, I am aware of my whole body.’ He or she practices like this.
  ‘...calm my whole body...
  ‘...feel joyful...
  ‘...feel happy...
  ‘...aware of my mental formations...
  ‘...calm my mental formations...
  ‘...aware of my mind...
  ‘...make my mind happy...
  ‘...concentrate my mind...
  ‘...liberate my mind...
  --from Anapanasati Sutta (MN 118), as translated by Thich Nhat Hanh


I'm not sure heart-beat alone provides the same level of insight into the state of body-mind as the breathing, even though it is related. If you think about the two from signal theory perspective, breathing is amplitude-modulated while heart-beat is (low) frequency modulated, so necessarily heart-beat is a lower-resolution signal. Although as Trungpa Rinpoche says, there is no reason it cannot be used as a secondary indicator, along with stomach and chest tension, face mimics, gait / body language etc.

More importantly, regarding your comment about it being difficult for you not to control breath. As per the above quotes, I don't think the point of (anapana) meditation is to watch breath without controlling anything, nor to regulate the breathing directly. The point is to use breath as a feedback for working with body-mind.