But next thing you know, you're hard on the floor, stiff as a board with several doctors above you, one checking your pulse, one feeling your forehead, one telling you to wake up, wake up.
That's what happened last Yom Kippur to Yael Rabin, now a sophomore at Shalhevet, who fainted half an hour into a 90-minute Neilah at Congregation B'nai David-Judea. She was feeling fine until suddenly she felt dizzy, blacked out and then woke up on the floor like it had all been a dream -- except that she was in throbbing pain all over her body.
"When I woke up, it was like someone had hit me with a wooden board several times," Yael said. Witnesses said she fell straight down and hit the floor so hard that services were stopped for several minutes.
Numerous doctors surrounded her and paramedics were called, and Yael and her family ended the holiday in the emergency room at Cedars-Sinai Medical Center.
"It was really hard the first few days" of recovery, Yael said. "There was so much pain everywhere, especially my neck."
Yael suffered from whiplash and a mild concussion. She couldn't walk and had to wear a neck brace for more than a week after. She couldn't play sports or participate in PE for the next few months.
Yael didn't feel any symptoms until it was too late, but if she had, she would have had Jewish law on her side in breaking her fast.
"In Yael's case, the fainting should have been avoided by breaking her fast because of the long-term health consequences that resulted when she didn't," said Rabbi Yosef Kanefsky of Bnai David-Judea, where two congregants fainted on Yom Kippur last year. Kanefsky advises at first eating small amounts that don't technically count as eating in Jewish law -- less than a cheekful of liquid and a kezayit (about the size of a cracker) every eight minutes. If that does not help, then one should fully break the fast.
From a medical standpoint, it turns out that the particular circumstances of Neilah, the closing service of Yom Kippur, add up to a kind of formula for fainting. According to Dr. Laurel Schramm, Yael's doctor and a pediatrician in Beverly Hills, dehydration coupled with extensive standing makes fainting all the more likely.
When teenagers faint it's usually because they're dehydrated, Schramm explained. Fasting contributes to dehydration, meaning that the body doesn't have enough fluid to send oxygen to the brain. A decrease in blood to the brain can cause loss of consciousness, or fainting.
Standing still for a long time makes matters worse by putting stress on the legs, causing blood to stay there and away from your head, she explained.
"When you're standing still, gravity pulls the blood down, and there's no muscle movement in your legs, no massaging the blood back up your body," said Schramm, who is Orthodox and fasts on Yom Kippur herself.
Coming at the culmination of a fast that started before sunset the night before, Neilah is the final, parting, pleading prayer when many Jews feel more connected to God than possibly any other time of the year. Maybe that intensity leads to fainting, too.
While shortening Neilah or abolishing fasting might seem like tempting solutions, that might ruin the emotional and spiritual impact of the day.
"Were it not for the fasting," Kanefsky said, "people wouldn't take the day half as seriously as they do. There would be no aura and sense of urgency around the day that exists now."
But, Kanefsky said, it is unnecessary to stand throughout Neilah while the ark is open.
"A common misconception is that standing is required when the Ark is open," Kanefsky explained. "In fact, one only has to stand when the Torah is moving, for example, when the Torah is being lifted after Torah reading."
Kanefsky usually announces this before Neilah every year, and he makes clear that anyone who feels that his health is in danger should eat the minimum quantities and can still feel he is following the law.
But if those things don't help, it's important to stay aware of the symptoms. If you suddenly feel cold and sweaty, or if you get dizzy and think you may faint, you should lie down immediately on the floor and raise your feet above your head, Schramm said.
Yael plans on fasting again this year, but she has a new awareness.
"If you feel sick, listen to what your body is telling you," she said. "God doesn't want you to get hurt."
Louis Keene is a senior at Shalhevet and on the staff of the Boiling Point, where this article first appeared.