Some people keep a dream log to keep track of what they dreamt of the previous night in order to help them with their daily decisions, but not many keep a log of their sleep pattern.

If you had a bad night sleep, you try and think back of what caused it and try and ensure it does not happen again. But sometimes it may not be obvious to pintpoint the sleep problem as it can be associated with what you ate or what thougths were on your mind during the day, etc.
If you keep a sleep log you may be able to see patterns emerging, which can help you solve the slepless night problems and get a more restful sleep in the future.

To make a sleep log easy to use, make a bullet point list of activities you did during the day and assign a score to them 1-10 ; 1 being little and 10 being too much, for example

exercise - 3

dinner - 4

cups of coffe drunk - 3

arguments - 5

The points above are just examples to help you set up a standard sheet for your sleep log, which obviously should be dated and can be quickly completed so you can look back over a week or month of info to help you spot patterns.