In most cases, a human being cannot simply think of nothing. It does not matter if you are male or female. When you are sleeping, your brain is going through some sort of analysis. In short, you are thinking of something or someone. So far, that is a crucial part of ones REM sleep. That is the state of sleep when you are having some sort of dream for the most part.
You are going to have problems sleeping if you are thinking about a lot of things. All of those thoughts running through your brain will send signals to your heart. In turn, your heart is going to beat faster. When that happens, you will be unable to sleep because of that fast heart beat. Also, that is going to cause you to have some burst of energy which makes it harder for you to sleep. Thinking about that, one could say that human beings are unable to think of simply nothing.
However, human beings are capable of thinking of nothing. It is not impossible. But, it all depends on the circumstances. As a martial artist, I can use my training as an example. When you are in a spar or a fight, you have to keep your head clear. Keeping a clear mind and thinking of nothing are virtually one in the same. In this respect, human beings are capable of thinking about nothing.
When I spared and practiced, I learned the importance of keeping a clear mind when going up against an opponent. When you keep a clear mind, you can react to just about anything. If you are thinking about something, you cannot defend yourself against an incoming attack. If you are thinking that your opponent is going to punch, you are anticipating a punch. Instead, your opponent throws a kick. You get caught off guard because you were anticipating a punch. If you are thinking that your opponent is going to throw a roundhouse kick, your opponent could throw a side kick. When you are thinking about something else, you are unable to properly defend. If you have a clear head, you can respond and defend against any attack.
While this is not about martial arts, I am using it as an analogy on that human beings are capable of thinking about nothing. In the case of combat, that is pretty much a necessity. If you are focused on trying to anticipate something, you will be caught off guard. I can use this one example: Before the start of this Karate class, I went to get a drink of water from the fountain. The instructor came out from the bathroom and tried to take me by surprise by throwing a front snap kick. I blocked it by reflex.
However, one can debate that I was thinking about getting a drink of water. But, someone else can debate that I was following my need to rehydrate. Virtually, I was not thinking about anything.
Another analogy could be the use of meditation. When one meditates, one has to keep a clear head for the most part. When you are thinking about nothing, you are pretty much at ease with yourself. Think of this as the "power of nothingness."
In these cases, it is not impossible for human beings to think of nothing. In the other cases, it is pretty much impossible for human beings not to think of nothing. If you are in a relationship, you could be thinking about your significant other. You could be thinking about sex.
You could be thinking about your finances. You could be thinking about whether or not you will be in your job the next year. There are so many things to think about. Overall, it all depends on your case. In certain cases, it is possible for humans to think about nothing. In most other cases, it is pretty much impossible to think about nothing.
Whenever I am sitting on a bench at the National Mall in Washington DC, one could think I am doing nothing. But, I am doing something: thinking. I am thinking about a lot of things. I could be staring at the sky or the vast ground that encompasses the National Mall. Still, I could be thinking about a lot of things. On that note, the National Mall is my favorite thinking spot.
I could be lying in bed trying to sleep. I am still thinking. I am thinking about a whole lot of things. Again, thinking in your sleep is essential to forming dreams. With that said, most cases make it impossible for human beings to think of nothing. There are rare cases in which humans can think of nothing.