Yawning is associated with some hormones that are released that briefly increase the heart rate and alertness. So actually the reason that one yawns when tired or bored is the body's attempt to keep you alert and awake — if only for a brief time.

Excessive yawning reasons include: Physical Causes—Fatigue, general tiredness, body temperature malfunctions, and stimulation of the vagus nerve can cause frequent yawning. Excessive yawning and sleep disorders are often correlated. Emotional Causes—Anxiety and depression can lead to yawning excessively.

There's little evidence to suggest that excessive yawning is caused directly by a vitamin deficiency. However, some vitamin deficiencies may cause fatigue, which often involves yawning. Not getting enough vitamin B12, for example, can lead to vitamin B12-deficiency anemia, which may lead to feeling fatigued.