Sleep Paralysis is to most a medical condition with general scientific explanations. It's phenomenon occurs when you are falling asleep or waking up. You are in between the two. When you sleep your brain keeps you from moving to keep you safe when dreaming. So when you actually experience sleep paralysis you are awake but your brain is not allowing your body to move speak or react. You can experience hallucinations. The best way to explain it is dreaming while your awake. It's also called Old Hag syndrome.
A psychic attack is when an individual or entity drains you of energy or life force. It's common for a psychic attack to occur when your sleeping because that's when you are the most vulnerable. When you experience a psychic attack as you sleep it's just like sleep paralysis. When it's very extreme you will suffer lasting effects throughout the day. But sleep paralysis/ psychic attack phenomenon occurs for multiple reasons. Here are some of the causes:
This is when a hag like entity sits on the victims chest. The victim is than motionless and experiences loss of breathe. This entity absorbs your energy and life force as it puts fear into you.
There are spirits known as vampire spirits. Instead of feeding on blood they feed on a persons life force. They very much do the same process as the Hag.
Succubus and Incubus are both sex demons but do the same thing as the Hag. This process happens before or after a few times of seduction and nightmares. When the victim fights back you experience intense pressure on your body. The victim becomes weak from the draining of energy and life force.
Many claimed abductees experience sleep paralysis. They also claim to experience it when aliens come for them.
A lot of times when you are new to astral projection there are moments when you about to project you go into shock. You begin to experience all the symptoms of sleep paralysis minus the hallucinations.
Many spirits/ghost/entities do the same thing. There are even moments when you cause it. We are very vulnerable in our sleep. A lot of people born with the ability of psychokinesis can create an entity during a time of stress and brings the attack to them.