What are possible causes for feeling dizzy and lightheaded when standing up from laying down and sometimes from sitting?

Referred to as. Orthostatic hypotension. When you go from lying down to seated, your heart has to pump blood against gravity to get it to the brain and blood also "pools" lower in the body. To overcome this, your heart rate increases and blood pressure needs to rapidly increase. Sometimes our bodies are slow to respond and this can lead to a temporary decrease in blood flow to the brain - that can make you dizzy.