hi ldz and welcome to CI!
yes, if you ONLY want header information (to conserve memory for instance) you can use a slightly different approach to do it.
DO NOT use the get_message() function; use message() instead.
1) set the message class that you want peeker to use to handle the messages. in your case it is the peeker_header class in the included peeker_header.php file. that class will ONLY access the header data for an email - not any body data.
2) get the email object using the message() function rather than get_message()
//peeker_header.php file must be in same directory as peeker.php (it is there in the default source distribution)
$this->peeker->set_message_class('peeker_header');
// get just the headers of the first email message
$email_object = $this->peeker->message(1);
if you have any problems check the documentation or post code here.
good luck and let us know how it goes.
cheers.