Read David Swenson's reply:
Myriad things can cause the general symptom of a headache—hunger, constipation, sinus congestion, allergies, holding the breath or forcibly breathing during asana practice, sunstroke, and many other possibilities. If the headaches are persistent and/or frequent, one should consult a physician as a precautionary measure to make sure that there is not some underlying condition.
In these cases, one of the first things I address is hydration. Dehydration can cause headaches, so it is important to keep enough fluid in the body. This is best achieved by setting an amount of fluid to drink and then sipping it throughout the day, rather than guzzling it down quickly. Water is the best hydration tool. Soft drinks, coffee, and even juice are not as effective as good, pure water. The amount of water appropriate for each person will be different depending upon how much they sweat, how active they are, and how hot or dry the environment is. People who live in a hot climate or practice a hot yoga style will require greater amounts of fluids to replenish what they lose. At least two to three liters per day is a good general amount.
If a headache occurs only during or right after practicing a posture, it’s likely due to incorrect alignment. If a headache occurs after back-bending, for example, it is often because the neck is bent too far back. It acts as a hinge, which produces a pinching effect. Elongating the neck could be the solution: Extend through the top of the head when doing a back-bending asana. Holding the backbend for too long could also be a source of the problem, so holding it for shorter lengths of time could be helpful. Encourage your students to think of backbends as a front opening rather than a back bending, and it will change the way they build the asana.
If the headache occurs after a Headstand, there is probably too much weight on the top of the head, and not enough in the arms. Standing on the head incorrectly can cause a compression of the neck and restricted blood flow that can cause a headache. If this is the case, it is best to return to the foundation of the headstand and build it up from the ground slowly, so the arms take the preponderance of weight.
Forward-bending asanas are not commonly associated with headaches, but if that's when the headache occurs, then—just as in the other asanas—look at the foundation of the posture and instruct the student to back out until the symptom is no longer present. Extend through the top of the head in all forward bends, twists, and even back-bending asanas. Remind your student that yoga should be a healing path, and if there are pains associated with the practice, then we should analyze if our approach and make adjustments as needed.
If sequencing is a concern, you may build up toward a full Headstand by using modifications first. Then be sure to rest in Balasana (Child's Pose) to allow the body and blood flow to regulate after the Headstand. The duration of Child's Pose may be determined by how long you remain in Headstand. The longer the Headstand, the longer the Child's Pose.
David Swenson made his first trip to Mysore in 1977, learning the full Ashtanga system as originally taught by Sri K. Pattabhi Jois. He is one of the world's foremost instructors of Ashtanga Yoga and has produced numerous videos and DVDs. He is the author of the book Ashtanga Yoga: The Practice Manual.