It can be common for people to struggle to remain awake on occasions during meditation due to three separate factors (see later on). Regardless of the explanations for having this propensity to doze off, the key resides in applying your awareness. Becoming aware is a core competency to be acquired through meditation, so, intriguingly, being aware is the key to not zoning out during your meditation activities.
The most common reason for people having a tendency to doze off during meditation is unsurprising a result of their hectic or frantic lives. If you have had a busy day or have struggled with stress, then you might identify that you often become drowsy during your meditation. This is simply because your body begins to destress from the pressures of your life as you perform the meditation activity. If you regularly experience this, then take time out to stand or splash some water on your face before proceeding. During the afternoon, lots of people experience an energy slump which can affect them when meditating. If that is the case, then it could be more beneficial for you to practice meditation in the morning when the body is awake. It is also common that a relaxed posture can contribute to the impulse to nod off, so take the opportunity to get started with some improved posture before you start your meditation. It does not mean that you need to sit in a contorted position, just becoming more aware of your body is the key.
A much more bewildering reason why you may become sleepy during your meditation is that your mind is proactively functioning to control your life. This might sound like a dichotomy that falling asleep is because of the activity of your mind. Yet it is effectively your mind which is applying control over you, hindering your ability to successfully practice meditation. In this way you might feel as though you are not succeeding, and that you cannot meditate. When the fact is, your mind is compelling you to be this way to ensure that it can preserve control. If you continue to meditate, you can strive to take back control from your mind, nevertheless be aware that your mind is most likely to strongly resist giving up control at any cost.
The final aspect which is seldom taken into consideration is that through the process of learning to meditate, you can awaken some of the inner energy of your true self. As the energy of your Life Force flows into your body, this can initially lead to a sensation of tiredness or even phasing out. This is simply because your body and mind are not accustomed to these kinds of energy apart from the minuscule quantities which are necessary to keep the human mechanism alive. If you are experiencing zoning out, then the key once more is the application of awareness. Being aware, as mentioned previously, is a cornerstone of meditation and it is never truer than when your Life Force energies begin to come to the fore via advanced meditation. As you apply your awareness, know these energies flow through the energy pathways (meridians) of your body. This knowing, through your awareness, will assist you to maintain your focus and not zone off.