While your sleep habits are probably the single most important thing about your daily routine that you can change, it's important to take everything into account.
Fetal position is a very common kind of side-sleeping, with some surveys saying that 41% of people choose it. However, there are downsides and benefits to this position. Traditionally, the "log" version of side-sleeping, with your legs reaching out, is healthier for your back than tucking your knees into your chest. Adding a pillow between your legs can ease the tension.
Back sleeping can be really hard to get used to, but it's worth the effort. When you lie on your back, your spine is supported perfectly by your mattress. Back sleeping also keeps your face wrinkle-free, because your face isn't pressed against a pillow all night.
While sleeping on your back will prevent back pain and muscle soreness, it can make your snoring and sleep apnea worse, so beware.
Your sleep habits depend on lots of different factors, some of which you can't help or change. But a small difference like sleeping on your back can have a big impact on your well-being!