Lie flat: wrong position = lie flat on the bed. That way the curvature is straight when you sleep. Straightening the lumbar spine increases the pressure on the lumbar joints and disc. At the same time, the muscles will also be tight due to changes in physiological curvature. So you'll feel stiff and sore at the waist.
Proper sleeping position = 1-2 pillows are recommended under both knees, allowing knees to bend. Can maintain lumbar physiology curvature not only so, reduce the compression force of lumbar, what also can promote lumbar muscle to be in when sleeping is loosened thoroughly.
Side: wrong position = lie on your side. The lumbar spine is pulled down by gravity, causing it to turn sideways during sleep. Just like the lumbar spine, which becomes straight when lying down incorrectly, the lumbar spine can also increase stress on joints and discs, leading to discomfort in the lower back.
Proper sleeping position = a pillow between your knees is recommended. This can maintain the physiological curvature of the lumbar spine and reduce the pressure on the lower back during sleep.
In addition to changing your sleeping position, stretching can also help reduce back stiffness and soreness.
It is recommended to perform the following stretching exercises before bed and after waking up to relieve back fatigue, stiffness and pain.
Supine buttock movement:
Lie on your back and twist your hips
Note: if the pain is high, reduce the amount of effort and hold time, and take a break between movements.
Thin backs were also mentioned in the question. This can be improved through abdominal and back muscle training. The following two exercises are recommended once a day.
Superhuman strength exercise (back muscle strengthening) : prone, knees apart and hip width apart. Extend your arms forward, palms down. Stretch your arms as far forward as possible and your legs as far back as possible. Raise your arms and legs simultaneously and extend them as far as possible. Hold for 5 seconds, slowly lower the limbs and return to the starting position. Repeat 5 times.
Scissor leg (deep abdominal muscle strengthening) : lie on your back and bend your knees, legs apart at hip width. Place your arms at your sides, palms down. Tuck in your stomach and relax your shoulders. Work your abdominal muscles while lifting your right leg, bending your knees until your thigh is perpendicular to your body (as shown in the picture above). Slowly lift your left leg to the same height as your right leg. Then start lifting and lowering your legs alternately. Repeat 5 times. Keep your belly tight during the procedure.