sometimes they are just sleepy but they are only sleepy like that when they are comfortable around you and don't see you as threat.

The persistent eyes-half-open state means they're relaxed and resting and yes, consider you a friend so they can afford to be relaxed and resting. They're usually also affectionate and very receptive to pets in that mode. Slow blinks can accompany the half-open-state, or a cat may use a slow blink to go from wideish to half-open eyes.

We don't understand cat facial expressions as intuitively as dogs

For example, what we would traditionally see as a grumpy or angry face (eyebrows down, eyes narrowed slightly) is the relaxed face of a cat

An angry cat's face on the other hand is wide open eyes, ready to attack back