AC 6110 - Because the famine was very grievous. (Genesis 47:13)

AC 6110
... truths and goods and the knowledges thereof
make the spiritual life of those who are in heaven,
for these are the celestial and spiritual foods
with which they are nourished.
These foods are given them daily by the Lord.
When it is morning with them, goods are supplied;
when it is noon, truths are supplied;
but when it is evening, goods and truths are lacking,
and this even unto twilight and the return of morning.
The angels are then kept in a state of appetite,
which is of such a nature
that they long for these things more
than those who are hungry on earth long for food.

[4] What is still more worthy of note is
that the shades of evening and the darkness of night
do not come from the Lord,
but from things that belong to angels, spirits, and men

[5] The reason why these alternations exist in the spiritual world
is that all who are in heaven may be continually perfected.