A page of head studies is fine. Ultimately what we're doing is solving a 3d spatial puzzle, which forces your brain to consider the relationships between the forms involved - whether it's a full body or a head, it's still going to serve its intended purpose. Just be sure to take advantage of the space available to you on the page - giving yourself that much more room all for a head means you'll have much more room to consider more nuanced elements of the construction than if you were using the same space to construct a whole body.

Also, while this is fine for a page or two, I still wouldn't rob myself of the opportunity to explore a wider variety of these puzzles (in terms of making sure you're still doing plenty of full body constructions as well) - just in case someone's reading this and thinking of just doing head studies.