Please explain "To Sleep," a poem by William Wordsworth.

Overall, someone who has been suffering from insomnia describes his difficulties in Wordsworth’s poem, “To Sleep.”
The title and the first two lines of the poem hint at the theme of sleeplessness even before it is clearly identified. For instance, the idea of counting sheep to put oneself to sleep is very common in the face of sleeplessness—even to a contemporary audience.
In the first four lines, Wordsworth describes the images that come into the speaker's mind as he tries to fall asleep.

A flock of sheep that leisurely pass by,  One after one; the sound of rain, and bees  Murmuring; the fall of rivers, winds and seas,  Smooth fields, white sheets of water, and pure sky... (1-4)

The speaker lists the things he has laid awake imagining, in an attempt to sleep. "One by one" gives reference to the act of counting sheep. "Leisurely" refers to the rolling gait and relaxed pace of the moving sheep (1-2). The speaker refers to the soothing sounds of rain, "bees murmuring" and waterfalls—"fall of rivers" (2-3). He also describes the visions of "smooth fields" (perhaps a part of an afternoon nap in the country), as well as a "pure sky," with no hint of storm and nothing in it that would cause anything but a quiet calm within, conducive to falling asleep (3-4).
In lines 5-8, Wordsworth's speaker explains that he has done all he can think of to bring sleep upon himself (5). However, he has had no success—he has remained awake all night long—until he finally hears the sound of birds that utter in the orchard, breaking the silence with their song in the early morning, we imagine just before dawn (5-6). Even the first cuckoo makes a melancholy: we can assume it is because the speaker is still awake to hear it, having had no respite from the day before. Note that the bird does not sing, but cries—it is a sound of distress (7-8):

I have thought of all by turns, and yet do lie  Sleepless! and soon the small birds' melodies  Must hear, first uttered from my orchard trees;  And the first cuckoo's melancholy cry. (5-8)

The theme of sleeplessness continues into the next three lines, giving the reader a deeper insight into the depth of the speaker's difficulty: for it is not just this one sleepless night, but the third in a row—

Even thus last night, and two nights more, I lay,  And could not win thee, Sleep! by any stealth;  So do not let me wear tonight away... (9-11)

With all the tricks he has tried to use, the insomniac has not been able to even steal—"Sleep! by any stealth..." (10)—any relief. In line 11 the speaker directly addresses sleep (as if it were a living, hearing thing), asking that it not allow him to remain in the same condition as he faces the approach of another night.
The poem's last three lines describe just how important sleep is:

Without Thee what is all the morning's wealth?  Come, blessed barrier between day and day,  Dear mother of fresh thoughts and joyous health! (12-14)

The "morning's wealth" (12) refers to all of the wonders of a new day. "Wealth" indicates that the morning is filled with things that are extremely valuable. He has already pointed out the sound of birds that welcome the day. They are a blessing at the beginning of the day, but only after a time of rest; the sound is very different when one has remained unable to experience rest, but instead has counted the long hours until the day breaks. What good, what joy is to be found in the coming of morning without rest from the night before?
The speaker praises sleep, referring to the "blessed barrier" (13), the thing that separates one day from another. He goes on to provide details of the benefits of a good night's sleep: "fresh thoughts" and "joyous health." (14)
Whereas the title might first lead the reader to believe the poem is about the pleasures of sleep, the content demonstrates that it is actually about the hardships created when one is unable to sleep; it points out the negative ramifications—most especially when the insomnia continues over subsequent nights.