There are not many things that are guaranteed in life. The old joke is that the only things that are guaranteed are taxes and death, but you could also add another: sleep. We all need it, no matter how young or old, no matter where we come from or what we do, we all need to sleep. There are a number of things that can affect our sleep, from internal, physiological issues such as an imbalance in neurotransmitters, through to changing light cycles in our external environment.
Two factors that can have massive effects on our sleep is pain and stress. For many, sleep is incredibly difficult because they are in chronic pain. In particular, people who suffer from back injuries and other conditions like arthritis find it hard to get to sleep because their pain is exacerbated just by lying in bed.
Likewise, stress, anxiety and depression can make it much harder to fall asleep and even when they do fall asleep their sleep will often not be as fulfilling as it should be. The problem is that our bodies are programmed to react to stress by staying awake, meaning that sleeping while stress is not just mentally problematic but physically difficult as well.