Above me is a on-site leader, and equally positioned to be (idk the correct term for his position), is the person that I have seen sleeping
From what I understand you mean that this person is above you, with their colleague being of the same seniority (so some kind of "co-leader" position?)- the important thing to ask here is: Are they responsible for you in any way, if not then whom or what are they responsible for?
If it's under the condition that they're there as "backup" in case they're needed, then this bumps down the "issue" list, but if they're actively meant to be working and supervising, then this is a problem.
Seeing as how this is a construction site, regardless of what's going on- who are they responsible for? I can only assume that they're effectively responsible for all the workers under them- not just in terms of the work they do but also health and safety of the workers, right?
sleeping whilst on work, now counting above 10 times. He sleeps in his car in hidden spots, even on his office wHILE I am on my neighbor office
Over what length of time has this been occurring? If we're looking at a period of several years, to months then this becomes an increasing issue, especially considering the above. Also, how long is this sleep for? Are we looking at brief "power" naps or several hours?
many of the workers at the construction site has noticed as well and are in fact commenting on it to me, and it feels like I am a bit in a weird position. I am also not sure whether my boss knows it.
At this point you need to think- has the other leader noticed as well, or just the workers under them?
You aren't in a weird position unless you have seniority over anyone, as they say, the chain of responsibility would somewhat obligate you into doing something- heck, traditionally speaking someone should do.
I don't want to generate any extra tension between me and my sleeping co-worker as we are in a critical phase of our project now towards its end
And yet, they sleep through the critical period, hmm?
So I am not directly willing to confront him directly about it, and at the same time, I don't know whether I should talk to my leader here on the construction site.
Well this is down to you, ultimately. If this person is jointly responsible for you and others- this other leader must surely be aware?
If this was somewhere safer and less serious, I'd say to leave this in the most non-confrontational and passive-aggressive fashion and say "Leave a note for the other leader or inform someone above them".
However, this is not the case as this is a construction site- this is dangerous to whatever degree, so something should be done.
Confront the person when nobody else is around, speak to the other leader or failing that, contact someone above them- but if you do, be prepared and follow through, bear in mind if the other workers will back you up if necessary and have some cards in your hand before you approach it.
If possible research what alternatives there are, depending on where you are in the world consult whatever safety regulations and laws are involved and who enforces them, take notes on the dates and times and if it comes to it, you're prepared.
Otherwise- forget it, get on with things, pray nothing happens and that they aren't responsible for your safety or wellbeing if it does.