My voice was already raspy/fading on the first song, and I thought, "Lord, how is this going to happen for eight more hours?" But my voice cleared up and as we got going it all became almost effortless in the Holy Spirit as His presence drew near. And, by the way, it's not that we're singing 500 songs back to back. There is what I'll call "improvised liturgy," where we flow with spontaneity into times of intercessory prayer for awhile, all while music keeps playing quietly in the background, then maybe we'll flow back into another song, then maybe we'll just sing "spiritual songs" where lyrics are improvised in the moment to express adoration in our own words, then flow back into a worship song, then flow into a time of reading and/or declaring Scripture, then maybe more prayer or another song. Also, for really lengthy sessions, I will take a 10 minute break after an hour or so to pick a new batch of songs, take quick bathroom break etc, though during the breaks we try to keep the vibe going by playing a worship CD softly in the background and/or do some quiet devotion time in the Word.
But in general "improvised liturgy" really makes the time fly fast. And the empowering, life-giving Presence of Christ gives alertness and strength even in the early hours of the morning.
Jesus, and the powerful communion we can have with Him, is better than coffee. :-)