if you would like to do something to open up your lungs to enable deeper breathing whilst asleep, an easy backbend like bridge pose, holding for 3 - 5 breaths, you could inhale into the centre of the chest at the heart centre, & exhale out of it...or go up & down into the pose, slowly, about 3x, & whatever version that you choose: this would open up the lungs. Then follow with reclining butterfly, soles of feet together, knees opened wide, lying on the back: observe the rise & fall of the navel with the breath, completely relax (go limp) the body with each exhale. If you would like to amplify the effects, have the exhale about 2x as long as the inhale, in these poses.
Another good combination is either cobra, up & down into the pose 3x, &/or hold the pose 3-5 breaths, then push back into child, for 3-5 breaths, relax with each exhale. Either of these 2 options are quite do-able for most people.
If you would like to sit & do some pranayama, breathing practices, then follow with Nadi Shodana: in left nostril, out right, in right, out left, being 1 round. Do 3-5 rounds. pause after inhale, completely relax on the exhale. Then another brief pause. If you can work up to exhales being 2x as long as the inhales, that would be beneficial.
Either follow with Bramari, or just choose Bramari or Nadi Shodana. In Bramari, one closes off the ear opening by pushing the bit of flesh that is in front of them, against the openings with the thumbs, as you exhale humming. Listen to the hums. Have your eyes closed Do 6 - 9. Extremely soothing to the body/mind/emotions/prana complex. Take your time, do everything slowly, letting your psyche know that it's rest & sleep time. Have the eyes closed for pranayama, & in the following techniques.
In bed, lie on your back if possible. Observe the gentle rise & fall of the navel with the breath. Inhaling, the navel rises, then a minuscule pause; exhale gently pull the navel towards the spine, gentle pause after. Release the abdomen to inhale, aim for longer exhales, then move into rise & fall of the navel with the breath, no effort required.
An alternative: observe the breath where it enters & leaves the nostrils. Personally, I think that if you are having trouble winding down & sleeping, it's best to get the focus away from the head! Another you could use is to have the sensation of your subtle body (not the physical body), expand with the inhale, return to it's physical shape as you relax with the exhale. Whatever technique that you use in bed - & you might prefer to skip the poses &/or pranayama - do it in a relaxed manner. The, as you start to feel sleepy, roll over onto your right side. The breath will predominate in the left nostril as you fall asleep, &, correspondingly, everything will relax as messages go from your brain to your system, telling you that it's rest time. When the right nostril predominates, as when you lie on your left side, the body/brain gets ready for action, so this is why we want the flow of breath to be in the left nostril as we go to sleep.