The first sentence, "I was sleepless," is grammatically correct and commonly used to express a state of being unable to sleep. It means that you didn't sleep or had difficulty sleeping. The second sentence, "I was a lack of sleep," is not grammatically correct. Instead, you can say "I had a lack of sleep" or "I experienced a lack of sleep." These phrases convey the idea that you didn't get enough sleep or had insufficient sleep. So, the first sentence is correct, while the second sentence needs revision for proper grammar.