Loungewear versus pyjamas: what is the actual difference
The distinction between loungewear and pyjamas used to be fairly clear. Pyjamas were for sleeping; loungewear was for the hours in between. Over the last several years, that line has blurred to the point where many women wear the same set for both. That is not a problem, but it does affect which fabrics and cuts make sense for your situation.
What pyjamas are actually designed to do
A pyjama set is designed primarily for sleeping. That means the fabric should be smooth against the skin to reduce friction during movement, the waistband should not dig in when you are lying down, and the cut should allow a full range of movement without riding up. Satin and washed cotton are the most common choices because both have low surface friction and breathe reasonably well.
The button-front shirt style that most people associate with pyjamas has a practical origin: it is easier to put on and take off without pulling fabric over your head, which matters when you are half-asleep. The wide-leg trouser is similarly practical, allowing movement without twisting.
What loungewear is designed to do
Loungewear is designed for the waking hours at home: making breakfast, reading, working from a sofa, watching something in the evening. The priorities shift slightly. You want something that looks intentional enough that you do not feel underdressed if someone rings the doorbell, and that is comfortable enough to sit in for several hours.
French terry, ribbed modal, and cotton jersey are common loungewear fabrics because they hold their shape when you are sitting and moving around, rather than draping loosely the way satin does. A hoodie-and-jogger set in brushed French terry is a good example of a loungewear piece that would be uncomfortable to sleep in but is very good for a long day at home.
When one set does both jobs
If you want a single set that works for sleeping and for the hours around it, a washed cotton sleep shirt or a ribbed modal two-piece is usually the best compromise. Both fabrics are smooth enough for sleeping and structured enough to look reasonable in the morning. The key is to avoid anything with a very loose waistband, which tends to twist during sleep, or a very stiff fabric, which does not soften with body heat.
Most people end up owning both eventually. But if you are starting with one, think about where you spend more time: in bed, or on the sofa.