While we sleep we keep our eyes closed and during this process some kind of energy with in the eyes gets generated and stored .Before opening our eyes we need to rub our palms to generate heat then open the eyes and look at our palms due to which the energy flows into our body and gets reused.
An event in Mahabharatha gives an insight of this.
Before Kurukshetra Gandhari gets afraid and asks Duryodhana to come to some place without wearing any clothes. Duryodhana accepts and gets started. In the mean time Krishna who is aware of what is going to happen makes fun of Duryodhana saying he is naked. Duryodhana feels shy and covers his waist with a cloth and then goes to his mom. In the mean time Gandhari who is not aware of the mischief done by Lord Krishna opens the cloth in her eyes and looks at Duryodhana. All the energy which got saved in her eyes for so many years gets focussed on Duryodhana ‘s body and it becomes like vajrakaya apart from the region he has covered.
As soon as the war starts Bhima and Duryodhana fight and Bhima could not understand the reason for his power. Krishna finally asks him to hit Duryodhana in the covered area post which Duryodhana gets collapsed.