Nectar is 80% water. The bees use this to feed themselves and they will also dry down the nectar to convert it to honey to eat later in the year. Honey is less than 18% water content. The bees fan their wings to evaporate the moisture and dry the nectar down. The sawtooth pattern in the plot of the line is a result of the bees drying off the moisture in the nectar overnight. This pattern will continue as long as the flowers and trees are blooming and continue to give nectar.
Last year the nectar flow began at the end of March and continued through mid-June, an unexpectedly long bloom period for Washington, DC. My four hives were so strong and the nectar flow went on for so long that I had to have two harvests - one in June and the second in July. I've only got one hive that is likely to make a honey crop this year so my fingers are crossed we get a nice long bloom!