Sometimes, ejaculation may happen while a you sleep. This is called a nocturnal emission or wet dream or mikreh lailah (nighttime occurrence).

This thread will be dedicated to discussion about the right perspective to nocturnal emissions, and is geared especially to young bochurim who have joined GYE to help deal with masturbation. 

Nocturnal emissions are normal
This is a normal part of puberty and a way for the body to get rid of old sperm. The testes constantly produce new sperm and as old sperm becomes unhealthy, the body gets rid of it through nocturnal emissions or reabsorbs it.

Is it a problem?
Do not worry about zera coming out at night. It is not an aveira unless the person has been intentionally looking at or thinking about inappropriate things before going to sleep. Even when it seems to be a result of intentional thoughts, it is far less serious than masturbation. Therefore on the 90 day chart, nocturnal emissions are never considered a fall. 

Do I need a Tikun for nocturnal emissions?
The Mishnah Berurah writes that "if you know that the keri has been caused by hirhurim, then you should strengthen yourself in learning Torah or in Tzedakah and Chesed". It also also suggested that you go to the Mikvah the next morning. If you find Mikvah to be triggering, you can just take a shower with Tisha Kabin.

כך הורו גדולי ישראל. ראה אגרות וכתבים לרבי שלמה וולבה ח"א, מכתב כד (הובא להלן בפנים). וכה הדריך מרן הסטייפלער במכתבו לבן ישיבה אחד (קריינא דאיגרתא ח"א, סי' קה): "והנני משתתף בצערך, ומקווה שהשי"ת יעזרך במשך הזמן להנצל מחטאות נעורים ר"ל. במה שנוגע על העבר אין לחשוב לעת עתה כלל כו'. עיקר גדול שלא להיות עצב ולא לחשוב אודות העבר כלל עד אחר נישואין, שאז יש ליתן לב לתקן העבר". וראה (שולחן מנחם להאדמו"ר מליובאוויטש זצוק"ל ח"ג עמ' קלה), "אודות העניין הידוע ותקונו, הנה בזמננו זה יש להסיח דעת מזה עד לאחר החתונה בשעה טובה ומוצלחת וכמובן ג"כ משו"ת צמח צדק שער המילואים ח"ב סי' סב, אשר המחשבה אפילו בהתיקון של הדבר עלולה ח"ו וכו' [לפעול בטבע הקרי כמו הרהורי עבירה]", עיי"ש. וראה כתר ראש לעיל "וכל שכן שמדאגה יוכל לבוא עוד יותר לזה ח"ו".

Nocturnal Accidents

In reply to your letter:

Generally speaking, the matter about which you write4 sometimes results from a lack of sanctity in the manner of speech. Often it is a result of the person’s physical weakness. Sometimes the person’s tefillin or mezuzos need to be inspected and changed.

Rectify all the above, and increase your energy in the study of Torah, Toras HaNigleh and Toras HaChassidus. May G‑d grant you success.

(Igros Kodesh, Vol. IX, p. 273)

Guard Your Eyes has an earlier article on this:

1) Always guard your eyes carefully during the day.

2) Guard yourself from lustful thoughts.

3) The Kitzor Shulchan Aruch offers advice on preventing keri like reciting the first four Psalms before going to bed, and not overeating before retiring to sleep, and avoiding spicy foods..... The Arizal advises the concentrated recital of the Shema before going to bed as both a preventative measure and as a rectification of the souls that were taken captive by the waste of semen in the past. He also advises wearing a tallit katan while sleeping as further protection.

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After Keri

If you can immerse in a Mikva, it is praiseworthy, but it is not an Halachic obligation.

If the Mikva is hard for you, it would be good to at least wash yourself off in a shower. If you can keep the water pouring over your head for a few minutes straight, it can be considered like a Mikva in some ways.

But even without washing yourself off, you are allowed to daven and learn Torah since we pasken that אין דברי תורה מקבלין טומאה - "The words of Torah do not become impure".

From the wording of this personal letter it seems that it was written to an older married person who was on a high level. Because for younger people, the likeliest cause cause is physical development and hirhurim throughout the day.

Just to put in into perspective. The Lubavitcher Rebbe often quoted the words of the Tzemach Tzedek (see reference in the earlier posts) in the name of the Baal Hatanya, that even if someone sees a keri on Yom Kippur it is not a cause for worry because nowadays it's not due to something spiritual, but because of hirhurim. The Tzemach Tzedek adds that even without hirhurim, the worry about having a keri on Yom Kippur in itself can cause a keri, and he concludes, "ולכן יסיח דעתו מזה לגמרי ויהי' שמח וטוב לב".

Rabbi Shlomo of Karlin wrote that a person should be more concerned if he has no fish for Shabbos, then if he has a Keri on Yom Kippur... (Beis Aharon Likkutim p. 144a) . 

Bottom line: Nowadays, being concerned about a keri is counterproductive, as thinking about it in itself increases the chances of it happening more frequently. Certainly when it comes to GYE members it should be treated as a very marginal issue. 

Of course, watching our eyes, and figuring out ways to deal with improper thoughts is something that needs to be focused on regardless. But that's a separate topic...