Master BKS Iyengar has said that one must practice asanas regularly. And, in case you don't have enough time for a full practice then at least do inversions.

The most important inversion is Sirsasana (the Headstand). Sirsasana is called the "King of all asanas" for a reason. It is the most powerful physical tool that we possess for overcoming the forces in the physical world that decay and age our body and mind. These forces show up as loss of mental power and clarity, loss of posture, shortness of breath, poor circulation and sluggish digestion. When all of these bodily system are working harmoniously then we remain youthful and bright for the extent of our lives.

Performing headstand.. or if you can't yet do headstand then a supported inversion...aids the bodily systems (circulation, endocrine,  respiratory, digestive, skeletal). The strain of gravity on our body and its systems is reversed by completely turning yourself upside down. Our relationship to gravity and worldly forces is changed completely and our body is nourished instead of depleted….

Headstand ensures a proper blood supply and stimulates the pituitary and pineal glands in the brain, glands that are responsible for growth and sex hormones. Our growth, health and vitality depend on the proper functioning of these two glands that control the chemical balance of the body.

Regular practice of Sirsasana makes healthy pure blood flow through the brain cells. This rejuvenates them so that thinking power increases and thoughts become clearer. Headstand stimulates the nervous system, increasing mental alertness and clarity. It is a centering, calming and soothing pose. People suffering from loss of sleep, memory and vitality have recovered by the regular practice of this asana.

Headstand strengthens the spine, neck, shoulders and arms. The muscular system of the abdomen and legs are toned. Blood and lymph fluid is relieved from the legs and ankles and with regular practice prevents the buildup of fluid in the legs and feet. Coupled with shoulder stand it is a benefit to people suffering from constipation. The lung tissue is stimulated, which relieves colds, coughs, tonsillitis, bad breath and palpitations.

By reversing the pull of gravity on the organs, especially the intestines, it helps to cleanse them and overcome problems of the liver, kidneys, stomach, intestines and reproductive system. Headstand increases gastric fire and produces heat in the body. When done properly, headstand helps the spine become properly aligned, improving posture, facilitating good breathing and reducing muscular stress. The weight of the abdominal organs on the diaphragm encourages deep breathing, which gently massages the internal organs. Sirsasana is used to treat asthma, hay fever, diabetes, headaches, anxiety and menopausal imbalance....

Like all things in life, the suggestion to get upside down should not be universally prescribed. There are certain contraindications that should be observed so as not to cause or exacerbate previous injuries or illnesses: unmedicated high blood pressure, some heart conditions, neck injuries, recent stroke, detached retina, glaucoma, and epilepsy are common issues that should be addressed before inverting. Talk with your doctor and teacher if you are unsure about your status. Additionally, the debate continues as to whether women on their “ladies holiday” should indeed take a vacation from inversions. I would suggest doing a little research for yourself, both in an academic as well as experiential sense—listen to your body and what feels appropriate as your move through your cycle. This might mean abstaining from or simply holding inversions for shorter periods of time—you are the ultimate judge.

Inversions are fantastic all year long, but in the winter months they may be especially beneficial. For many of us winter comes with it chilly days and more time spent indoors. There is a natural biological tendency to draw inwards, conserve energy, and move more slowly.

So while there is certainly validity to the idea of paying attention to one’s internal seasonal moods and following suite, for many, the world around them does not abide by the same clock. And so while it may be more of a challenge to incorporate inversions into one’s practice during the quiet months, doing so may serve as a natural-caffeine-free way to bust out of the wintertime doldrums and access hibernating stores of internal energy….

Article sources:

http://lynnrescigno.blogspot.com/2008/12/sirsasana.html

http://empowerpeace.typepad.com/empowerpeace/2011/03/the-king-of-all-asanas.html

http://yoganonymous.com/10-benefits-of-yoga-inversions/