Write a function to insert a sentence in a text file, assuming that text file is very big and can't fit in computer's memory.

Let the file "insert.txt" include the following sample text:

Bees hum
leaves rustle
Waves crash
nature's voice whispers

def insert_sentence(file_path, sentence):
    file = open(file_path, 'a')
    file.write(sentence + '\n')
    file.close()
insert_sentence("insert.txt", "life's essence glimmers")

The "insert.txt" file includes following text:

Bees hum
leaves rustle
Waves crash
nature's voice whispers
life's essence glimmers