During daytime usually people are busy in their work.
Anxeity occurs in night due to exccsive thinking on same process. Hence worsens.

As my esteemed colleague has suggested, the anxiety at night could be because you are being distracted away during the day time by work and hence at nights you are thinking/brooding about things which cause you worry.

Anxiety is all about "fear" and hence you need to ask yourself what is it that you are anxious/fearful/scared of? What is causing you this anxiety?

There are some physical causes of periodic anxiety but they are rare. If there are no psychological reasons why you should be worried/anxious at night/when alone then it would be better to see a general physician who may rule out episodic anxieties.

If there are some psychological/social/past experience/current family/financial/other stresses and if the anxiety is not too sever/too disabling/not disturbing your life much, then simple breathing excercises/pranayam/yoga etc can help.

But if this anxiety is causing too much trouble, then please do not hesitate to consult a psychiatrist as long term anxiety is very difficult to treat than anxiety caught in the beginning

Good Luck