If tired, try going to bed earlier if this is possible. Since I started going to bed at 10pm, I am able to wake at 4am with little grogginess. Whereas when I went to bed at 11pm, I would want to sleep in until 6am or 7am and I would feel groggy upon awakening. Now that I've made early sleep a habit, the difference is stark.
As they say, "an hour before midnight is worth two after". I've read that it has to do with circadian rhythms and electric lighting. Going to bed earlier is closer to what our ancestors would have done before the advent of electric lighting. Thus, it is more natural and healthier for the body.
Also, see these notes I made summarizing Anguttara Nikaya 7.61 dealing with remedies for drowsiness/dulness.
• AN 7.61: How to remove drowsiness, in order of most to least preferable method
o Don’t attend or concentrate on the object that you were attending to when you became drowsy.
o Ponder, examine, and mentally inspect the Dhamma as you have heard it and learned it.
o Recite in detail the Dhamma as you have heard it and learned it.
o Pull both ears and rub your limbs with your hands.
o Get up, rub your eyes with water, survey all quarters, look up at the constellations and the stars.
o Attend to the perception of light; undertake the perception of day; as by day, so at night…as at night, so by day; develop a mind imbued with luminosity.
o Walk back and forth, perceiving what is behind you and what is in front, with your sense faculties drawn in and your mind collected.
o If all else fails, take a very short nap and get up quickly when done, thinking: ‘I will not be intent on the pleasure of rest, the pleasure of sloth, the pleasure of sleep.’
Then the Lord said to the monks: 'Now, monks, I declare to you: all conditioned things are of a nature to decay - strive on untiringly.' These were the Tathagata's last words.
-DN 16, trans. Maurice Walshe
'Whatever should be done, bhikkhus, by a compassionate teacher out of compassion for his disciples, desiring their welfare, that I have done for you. These are the feet of trees, bhikkhus, these are empty huts. Meditate, bhikkhus, do not be negligent, lest you regret it later. This is our instruction to you.'
-SN 43.1, trans. Ven. Bhikkhu Bodhi