Is the use of Mindfulness of breath for falling asleep a "bad" practice? Might such a practice invite torpor during sitting meditation?
If you are using meditation to go to sleep:
- You are inducing a hindrance. If this becomes habitual it will block your progress.
- Also when meditating you should be alert and hence reduce the hindrances and sleepiness. If you are not falling asleep it not an issue if you are meditating. It is said a Yogi never sleeps.
As Suminda commented, the risk is that you build a habit of becoming sleepy while meditating.
However, if you are having trouble falling asleep because your thoughts are racing, I would think that it's entirely appropriate and beneficial to use something like breath meditation to bring calm and focus to your mind.
Once this is accomplished, it's probably worthwhile to intentionally stop this practice, and allow yourself to fall asleep naturally. If your thoughts begin to race again, then repeat the process.
I do this often and don't find it problematic. I do the same thing to bring focus and calm before athletic events, public speaking, etc.
If you're feeling excessively sleepy while meditating, sleep and then come back to meditation. If you're feeling excessively "meditate-y" while sleeping, get out of bed and meditate! It might be a chance to level up!
In general, when sleeping, don't try to meditate or vice versa because sleep itself is a Hindrance and conflicts with meditation and vice versa (meditation is Awakening and is the opposite of torporous sleep). By default try to avoid a Hindrance when you can unless necessary because sleep is actually a fault, a flaw, of untransformed consciousness and one day, when you are fully enlightened, you will not need it at all. This is actually a valuable rule for the other Hindrances as well. Anyway, back to the topic of sleep.
This being said, there is an entire awesome world of dream cultivation and lucid dreams where "your cultivation power increases tenfold" according to Tibetan Buddhism and from my own personal occasional experiences with meditation within lucid dreams and also causing the lucid dream themselves. And how does this happen? By breaking that general rule... mixing sleep with mindfulness.
Now this is a complex topic with lots of differing perspectives but I'd suggest trying things out because lucid dreams are worth it. You can emerge from them feeling, super-conscious, awesomely self-aware, extremely blissful and joyful and self-integrated... basically if you are not able to get samadhi in your normal consciousness, if you experiment with mindful sleep you can BUMP INTO samadhi states with the multiplicative power of lucid dreaming.
My suggestions, after over 15 years of experimenting with meditation and lucid sleeping, is this:
- the first half of your sleep (3-4 hours, "core sleep") focus on the torpor and just fall asleep
- the second half, practice any meditation technique AS you fall asleep and decide to be aware through the process! Declare your intention and be aware as you fall back to sleep. ("WILD technique")
If you want to learn more (there is a treasure trove on forums and the internet) look up the above terms in quotes.