This little poem named A Dream Blake sometimes included in Songs of Innocence and sometimes he put it in Songs of Experience. He was aware of the dream state and how it could transform one's realities.
Original in Yale Center for British Arts Songs of Innocence
Plate 26, Copy F
Songs of Innocence, Plate 26, (E 16) "A Dream Once a dream did weave a shade, O'er my Angel-guarded bed, That an Emmet lost it's way Where on grass methought I lay. Troubled wilderd and folorn Dark benighted travel-worn, Over many a tangled spray All heart-broke I heard her say. O my children! do they cry Do they hear their father sigh. Now they look abroad to see, Now return and weep for me. Pitying I drop'd a tear: But I saw a glow-worm near: Who replied. What wailing wight Calls the watchman of the night. I am set to light the ground, While the beetle goes his round: Follow now the beetles hum, Little wanderer hie thee home."
So this dream of which he wrote was of an emmet (ant), a glow-worm and a beetle. Hidden in the foliage of Blake's illumination you may find these little creatures. You will also see the London night watchman, the glow-worm in his human form carrying a lantern for light, in the bottom right corner of the picture.
Now if you look further at the words, you read of a weary traveler heart-broken as she weeps over her children. If you are as familiar with the Bible as Blake was, you may be remembering Jesus, weary from walking, as he approached Jerusalem for the last time. He heard the cry of Jerusalem's children and offered a light and a word which could lead them home.
The poem led me to thoughts of Jesus and his wanting to gather Jerusalem's children as a hen gathers her brood under he wings. From there my attention was drawn to the words, "Blessed is he that cometh in the name of the Lord," presented as a quotation. I followed that lead to First Samuel where I read of the youthful David saying, "but I come to thee in the name of the LORD of hosts".
 The same day there came certain of the Pharisees, saying unto him, Get thee out, and depart hence: for Herod will kill thee.
 And he said unto them, Go ye, and tell that fox, Behold, I cast out devils, and I do cures to day and to morrow, and the third day I shall be perfected.
 Nevertheless I must walk to day, and tomorrow, and the day following: for it cannot be that a prophet perish out of Jerusalem.
 O Jerusalem, Jerusalem, which killest the prophets, and stonest them that are sent unto thee; how often would I have gathered thy children together, as a hen doth gather her brood under her wings, and ye would not!
 Behold, your house is left unto you desolate: and verily I say unto you, Ye shall not see me, until the time come when ye shall say, Blessed is he that cometh in the name of the Lord.
1 Samuel 17
 And the Philistine came on and drew near unto David; and the man that bare the shield went before him.
 And when the Philistine looked about, and saw David, he disdained him: for he was but a youth, and ruddy, and of a fair countenance.
 And the Philistine said unto David, Am I a dog, that thou comest to me with staves? And the Philistine cursed David by his gods.
 And the Philistine said to David, Come to me, and I will give thy flesh unto the fowls of the air, and to the beasts of the field.
 Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the LORD of hosts, the God of the armies of Israel, whom thou hast defied.
 This day will the LORD deliver thee into mine hand; and I will smite thee, and take thine head from thee; and I will give the carcases of the host of the Philistines this day unto the fowls of the air, and to the wild beasts of the earth; that all the earth may know that there is a God in Israel.
 And all this assembly shall know that the LORD saveth not with sword and spear: for the battle is the LORD's, and he will give you into our hands.
 And it came to pass, when the Philistine arose, and came and drew nigh to meet David, that David hasted, and ran toward the army to meet the Philistine.
 And David put his hand in his bag, and took thence a stone, and slang it, and smote the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth.
 So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David.
So you can focus on the glow-worm acting as a watchman for the ant, or the London night watchman providing a light and humming sound as he patrols the dark streets of the city, or you can focus on the son of David who came into the world to enlighten every man. Blake offers a dream, and a dream state, and an opportunity to transcend ordinary consciousness and mount your own fiery chariot.
A Vision of the Last Judgment, (E 560) "If the Spectator could Enter into these Images in his Imagination approaching them on the Fiery Chariot of his Contemplative Thought if he could Enter into Noahs Rainbow or into his bosom or could make a Friend & Companion of one of these Images of wonder which always intreats him to leave mortal things as he must know then would he arise from his Grave then would he meet the Lord in the Air & then he would be happy"
Here are more Biblical passages apropos for a full understanding of the poem:
 In him was life, and the life was the light of men.
 The light shines in the darkness, and the darkness has not overcome it.
 There was a man sent from God, whose name was John.
 He came for testimony, to bear witness to the light, that all might believe through him.
 He was not the light, but came to bear witness to the light.
 The true light that enlightens every man was coming into the world.
 He was in the world, and the world was made through him, yet the world knew him not.
 He came to his own home, and his own people received him not
 And when he was come nigh, even now at the descent of the mount of Olives, the whole multitude of the disciples began to rejoice and praise God with a loud voice for all the mighty works that they had seen;
 Saying, Blessed be the King that cometh in the name of the Lord: peace in heaven, and glory in the highest.
 And some of the Pharisees from among the multitude said unto him, Master, rebuke thy disciples.
 And he answered and said unto them, I tell you that, if these should hold their peace, the stones would immediately cry out.
 And when he was come near, he beheld the city, and wept over it,
 And as they led him away, they laid hold upon one Simon, a Cyrenian, coming out of the country, and on him they laid the cross, that he might bear it after Jesus.
 And there followed him a great company of people, and of women, which also bewailed and lamented him.
 But Jesus turning unto them said, Daughters of Jerusalem, weep not for me, but weep for yourselves, and for your children.