Though antar kumbhaka is a seemingly simple and straightforward practice, it does have vast repercussions on the body and mind. You must be alert to any adverse reactions. These can take a variety of forms from excessive spots on the skin to the inability to sleep. In the first case the kumbhaka is rapidly purging the body of inherent impurities; because of the rapidity, the poisons are causing numerous spots to erupt on the skin as they are expelled. In this case reduce or even stop the practice of kumbhaka for a short time to allow the body to cleanse itself more slowly. In the second case, kumbhaka is in a sense overcharging your body and mind above its normal level of activity. Reduce or stop your practise for some time. There are many other possible adverse reactions that can occur. Be alert to them and if you need advice seek the guidance of a competent yoga teacher.
The usual reason for any kind of adverse reaction is generally too much practice in the early stages. Be moderate in the time you spend daily doing pranayama, especially kumbhaka. For the first six months you should do no more than 10 minutes. If you want to do more and your constitution is adequate then probably you can with great benefit. If you are not sure about your practice then once again we advise you to consult someone with experience.
We emphasize that the duration of antar kumbhaka should be slowly increased without any strain, so that the body mechanisms are slowly accustomed to the new level of functioning. If you have not practised stages 1 and 2 of nadi shodhana pranayama we strongly recommend that you do them for a month or so before you attempt serious practice of antar kumbhaka.
In accordance with other forms of pranayama the best time to practise is after asanas and immediately prior to meditation practice.