Elevating your knees while you're sleeping is a good thing and helps improve your blood circulation and improves heart health.
Elevating your legs is also a good thing especially when you have issues with blood clots or have heart disease or other blood related health issues.
The legs being elevated keeps blood circulating to your to your heart and brain much easier than it would if your legs are not elevated.
When you're resting in a recliner or laying on the couch it can be a good thing to prop up your legs or knees which helps keep your blood circulating.
When sleeping or resting you can elevate your knees by propping your knees up above the level of your heart. In the daytime, lie on your sofa or bed and place a stack of cushions/pillows under your lower leg/foot.
At night, raise the foot of your bed.