As a general rule, the point of Amud Ha’shahar – around 72 minutes before sunrise – marks the beginning of daytime with respect to Halacha. As such, Misvot which one is required to perform at night cannot be fulfilled after the point of Amud Ha’shahar.
However, Hacham Ovadia Yosef, in Halichot Olam (Parashat Vayera), notes three exceptions to this rule, Halachot which are relevant specifically at night, but apply even after Amud Ha’shahar.
The first is the Misva of Birkat Ha’lebana, the special blessing recited in the beginning of the month over the renewal of the moon. Although this Beracha must be recited specifically at night, Hacham Ovadia rules that one may recite it even after Amud Ha’shahar, as long as it is still dark enough that one derives benefit from the light of the moon. Therefore, one who did not recite the Beracha during the night can still recite it after Amud Ha’shahar, as long as he derives benefit from the moonlight.
Likewise, one who did not recite the Beracha of "Boreh Me’oreh Ha’esh" over a candle on Mosa’eh Shabbat may recite it even after Amud Ha’shahar, as long as it is still dark enough to derive benefit from the candlelight. Even though the day has technically begun, one may nevertheless still recite the Beracha over the candle, since the Beracha is recited over the benefit derived from the light.
The third exception noted by Hacham Ovadia relates to marital relations, which, under certain circumstances, should not take place during the day. Hacham Ovadia rules that with respect to this Halacha, the night does not end at the point of Amud Ha’shahar, and thus marital relations remain permissible even after that point.
Summary: Generally speaking, Halachic nighttime ends at the point of Amud Ha’shahar, approximately 72 minutes before sunrise. Nevertheless, one may recite Birkat Ha’lebana and the Beracha over the candle on Saturday night even after this point, as long as it is still dark enough to benefit from the moonlight or the candle. Additionally, although one should not engage in marital relations during the day under certain circumstances, this is permissible after Amud Ha’shahar.
