Warning letter on Sleeping while at work

Dear All,
Some of my employees were sleeping at work. I want to issue warning letter to them, please help me by sharing draft of the letter.
Regards
Mahendra

Before issuing warning letter, please try to understand 'why they are sleeping at work'
1. What are they doing after working hours? working somewhere else?
2. Low energy?
3. Boring work?
4. Is this affecting the quality of work?
5. Why other employees are allowing these guys to sleep at work?
6. What is the supervisor's role? Is he also sleep at work?
7. Do you think just by warning, they will stop sleeping?

Sir: Draft warning letter is given below
To,
-----------------
You were found sleeping at ...... (time) on xx/xx/xxxx (date) at work place and has thus neglected your duties. Sleeping while on duty endangers not only oneself but it could also be a health hazard for others as well.
Sleeping while on duty and negligence of duties are misconducts under the Rules/Standing Orders but a lenient view is taken this time. You are severely warned to be careful in future and any repetition of the act would be seriously dealt with.
The receipt of this memo shall be acknowledged.
XXX
Supervisor