The Fluffy Stuff
Since I'm an English teacher, you're probably expecting me to tell you that summer reading is important because it exposes you to great literature, encourages you to read for pleasure, and helps mold you into a life-long reader. All of these things are true, and yes, I think they're important, but I'm not naive enough to suggest they're a sufficient answer for why you should devote so much of your free time to reading when you could be splashing around at the pool or riding roller coasters. Summer reading and high school achievement are linked in a variety of ways, some more tangible than others, but all worth looking into.
If the fluffy stuff wasn't cutting it for you, not to worry. There are plenty of very logical and practical benefits of summer reading. The links between summer reading and high school achievement aren't as romantic or frivolous as you might think. Completing your summer reading will directly benefit your grades, vocabulary, reading comprehension, and writing skills.
Let's start with grades. Assuming your school has given you an actual assignment for summer reading–as opposed to simply suggesting that you read–blowing it off will have a major effect on your first quarter English grade.
Let's say you were instructed to read Charles Dickens's A Tale of Two Cities over the summer. You could blow it off. However, not only will you have to deal with your parents nagging you all summer, but come September, you'll also have to deal with whatever assignment your English teacher throws at you. Unfortunately for you, your teacher will probably count the summer reading project as a "major grade" which means it's worth about the same as a test, essay, or other big project.
In my experience as a teacher, it's very hard for students to dig themselves out once they receive a zero on a major assignment because there are usually only a few more of them throughout the quarter. Okay, so let's say you blow off the reading, but then try to complete the project anyway, to save yourself from that deadly zero. Anything you earn is better than nothing, but chances are you won't be able to impress your teacher enough to earn an A or even a B on the project, since your knowledge and understanding of the novel are so shaky. So you're still starting the year without reaching your potential.
What's more, all that time and effort you're putting in to figuring out what the heck your teacher is asking about is distracting you from your other classes. It's easy to bomb an algebra test when you stay up all night desperately trying to finish your paper on "Dickens' Attitude toward the French Revolution" with relevant passages from the novel serving as your evidence. Hey, nobody said high school was easy.
Even if you weren't given a specific assignment, summer reading still affects your achievement in high school in a number of ways. A challenging novel is almost certainly going to expose you to new vocabulary, and you're much more likely to retain new vocabulary when you learn it in context, not off a random list of words. You might not even realize you have your English teacher to thank when you recognize "pernicious" on the SATs, but there's a big chance you remembered it meant "hurtful" because you read it in A Tale of Two Cities.
In a similar vein, struggling through Dickens's prose is not a bad thing. Really. It means that you'll be in good shape to read other challenging works, like those by Shakespeare and Faulkner, because you've had practice with reading comprehension. And believe it or not, reading great writers subconsciously helps you become a better writer, too. You might not notice it, but you pick up on Dickens's varied sentence structure, word choice, and sensory imagery as you peruse his novel. Don't be surprised if you start seeing beauty in your own writing after reading the masters.
Avoiding the Summer Slide
There's also plenty of research that shows that students can regress quite a bit over the summer if their minds are left idle. Studies show that you could lose up to two months' worth of instruction–meaning you'd come back in September with about as much knowledge and understanding as you had in March or April, losing the equivalent of May's and June's hard work and effort–if you're not reading or pursuing other educational activities in the summer months. That's a scary thought!
Students who read over the summer are keeping their minds active, so not only do they gain whatever learning their book provides (i.e. the vocabulary, reading comprehension, and writing skills described earlier), they're actually far less likely to lose what they learned the previous year. In a sense, reading a book or two over the summer could be enough to put you at the head of your class. That certainly seems worth the effort to me!
Keep in mind, too, that students who read over the summer are significantly more likely to continue to read throughout the school year, despite their more limited time in these months. All those books add up to quite the advantage over non-readers.
The Benefits Beyond High School
Hopefully I've convinced you that it really is in your best interest to do your summer reading. Heck, I hope I've convinced you to embrace it. But if you need more convincing, I'm still here. As I said at the beginning–back in the fluffy stuff–one of our hopes in assigning summer reading is "creating life-long readers."
Truth is, this isn't fluff. There are serious rewards to being a life-long reader. First, you'll be infinitely more interesting. It might sound silly, but when you're in college or among adults, you don't want to be the only one in the group who doesn't get a joke because the literary allusion is foreign to you. Plus, it's so nice to have conversations with people who have an overlapping literary background with you. There's plenty of fodder for conversation there. And you'll know so much more than you would if you'd spent all that time doing something more trivial.
If that's still not enough for you, though, I encourage you to check out Caleb Crain's 2007 article for The New Yorker, Twilight of the Books, which discusses the alarming decline of reading in America. The whole article is worth reading, but it's got some shocking statistics that you can't miss. You might be surprised to learn that people who read regularly are also more likely to make more money (okay, that makes sense), vote (what?), and exercise (WHAT?). People who read take care of themselves, and I would venture to guess that that's something we all want for ourselves. So remember as you're slowly plugging away through Dickens (or whatever else was assigned to you), that it's worth it in both the short and long term.